I really like that the middle button starts the scroll mode. Honestly, for about 2 years I hated it, but then I decided to use it to read for a while and now I love it. Now I do use it all the time just to “look around” on a web page.
It’s especially great because on badly designed pages you can look left and right too. It feels much more natural, though I still use the scrollbar when I am “forced” to.
It’s all about what you are used to.
I’m totally addicted to wheel-clicking links to open them in a new tab wheel-clicking to close tabs in IE or FF. But I’m equally addicted to the 4th mouse button on my MS Intellimouse that sits under my thumb and is used for going up a folder in Explorer or back a page in browsers.
I use the modal autoscroll mode quite often too.
I almost agree with Hinek’s suggestion A. However…
Since grab (move) and activate are probably the most often used mouse-button actions (can anyone find research to back this up?), I believe it would be best to map both of these to the left mouse button, since it is the most comfortable and natural one to click. Having to right-click too much would probably wreck your wrists after a while.
Thus, I would suggest the following:
Left click : Activate
Left hold: grab/drag (to MOVE, if applicable)
Right click: Select
Right hold: grab/drag (to COPY, if applicable)
Middle click: context menu
And possibly, in case middle clicking is not always available, or too difficult with the combined scroll wheel/middle button:
Right click on previously activated object: context menu.
Of course, all of this discussion is moot, since most people are already accustomed to the various mappings in use today.
Now, the keyboard, then, is of course another thing that is REALLY in need of a re-thinking.
I find the macbook pro’s track-pad functionality to be the next best thing to a mouse, sometimes better.
The ‘two finger’ scroll conversion beats the mouse wheel by a mile, and two finger-click to right click is decent. When I use a windows track-pad, I always find myself accidentally right clicking, in this usage the single button is much better. The track-pad is easy to use with either hand depending on which side of the keyboard I might want to use with the other.
As more people move to laptops, use of trackpads will become more and more common; and I think Apple’s success in the laptop business is due in part to their usability advantage there.
I’ve used it when my scroll wasn’t functioning to well, but compared to the other buttons, it redundant in my mouse usage.
However on a FPS game once, I did bind it to a grenade throw. 
Shame on me : I discover the “please browser, open this link in a tab” third-button behavior with this post !!! Maybe because I use it mainly for copy/paste, in such an intensive way that with Windows I miss it quickly, making me angry.
""In the UNIX and X Windows world, the middle button has also meant paste since way, way back in the 1980s. I can’t find any evidence of this behavior on Windows or the Mac, however. "
On my OS X 10.4.11 working in terminal pastes selected text at the cursor position with the middle-click."
This is because OSX is Unix …
And thank you I have now turned off autoscroll … I learn something useful from reading this blog quite often …
This goofy problem luckily sorta has a solution already: mice that have programmable buttons and application-sensitive settings.
Blender uses the middle button for several things.
Using Vista x64 and the Microsoft Intellipoint software, my middle mouse button brings up the task switcher. I love it. No matter what app I am in or what the mouse is on, it does it.
I would suggest middle-click for edit/enter/open.
These concepts are much needed and often used but have no clear standard, except for the badly designed double-click or sometimes the enter key.
•Browser: Middle-click to open the link in a new tab (already used so).
•Spreadsheet: Middle-click to edit the cell.
•File explorer: Middle-click to open the file/folder (already used so sometimes).
•Layout designer: Middle-click to edit the inside of an object.
It could also be considered as “Forward” and complement another navigation feature: “back/exit”.
Left-click could them focus more on use/activate.
Right-click stays on select/properties.
I agree that mouses would be better with a trackball for scrolling or sub-selection actions.
I somehow never got to use Middle-click to scroll, be it holding it or switching mode… yet I don’t know what is wrong with it.
As for copy/cut/paste, these definitely need fast access… but I don’t think a dedicated mouse button is correct.
My middle click is set to the app switcher in OSX. It is probably the single most frequently used button other that left click that I use all day. My other button is set to expose, and with the option and control and command keys, I use expose all windows, desktop and spaces. Also, option middle click is set to dashboard.
At this point, the ‘additional’ buttons have been left unstandardized for so long, I think the only way to go is to make them fully customizable. Just try to take away my middle-click app switch. I dare you.
I like auto scroll, I was even using it as I scrolled past that graphic of yours. 
I mostly use it on my laptop to scroll at reading pace - using L+R middle click emulation works fine - so I don’t have to keep touching the pad. (although the scroll strip is fine)
I think middle mouse should just keep doing what it’s doing - standardised enough.
Mice with hard to press middle buttons are annoying.
My current mouse doesn’t have middle click actually. Clicking the scroll wheel switches the wheel from a “geared” mode to frictionless…
Plenty of other buttons though…
Try A4 tech SWOP 80 and iOfficeWorks software. I use 1,2 buttons in standard mode; 3,4 copypaste; 5,6 backforward in internet browser mode; middle button mean alt+F4; 7 - iOfficeWorks button run menu with most useful functions like cutcopy, zoom±, pageuppagedown for button 3,4. Sometimes when I use chat window my thumb travel from mouse to numeric enter for sending messenges and “Next” buttons 
how about leaving the middle as it is?
a button that is free for developers to implement functionality that is useful for their specific app.
I love the mid button. I love it for its multiple uses such as pen/close tabs, autoscroll and transform 3d spaces (maya, 3d studio etc).
I love the forward/back button under my thumb on my Logitech. I hate browsing the web without it. I think this should be a new standard as well.
As a person who configures computers for a wide variety of people, about 75% of the Windows users are simply unaware of the Context Menu that the left button provides. As far as these users are concerned, there’s only a single button on the mouse.
In fact, the second button is actually confusing to them, especially when they switch the mouse from one side of the computer to the other. So, maybe Apple had a point of creating a single button mouse. You don’t have to keep telling users “No, press the other button!”.
Personally, I am in the minority. I can’t stand a single button mouse, but that might be the real genius of the Apple Mighty Mouse: In standard configuration, it is a single button mouse, but if you know what you’re doing, you can configure it to be a two, three, or five button mouse. For my wife, the Mighty Mouse is a single button mouse, for me, it is a multi-button mouse with a scroll wheel.
I also decided to settle once and for all whether the Mighty Mouse sucks or rules the correct way: A Google Fight. At first, I simply did “Mighty Mouse Sucks” and “Mighty Mouse Rules”, but ended up with a lot of hits to Mighty Mouse cartoons. So, I added the word Apple to the query. “Apple Mighty Mouse Sucks” brings up about 64,200 hits while “Apple Mighty Mouse Rules” brings up 78,200 hits: http://snipurl.com/22ulc [www_googlefight_com].
Would the people that complain about Jeff’s self-linking please shut the hell up about it? No one gives a damn - it’s what he’s always done, and many enjoy it. If you don’t like it, then bugger off and find another blog to read.
I’ve always set my mouse to task switching functions. In OSX Leopard it is set to spaces(with all apps i use locked into a space), KDE it is set to switch to the next desktop in the loop (in kde4 it will be set to launch the desktop manager), in tiger it was set to expose and in windows i just had it change to the next running program. If i were to design my own desktop environment though it would certainly bring up a pie menu arround the mouse with all the programs running going arround it.