(Last updated: July 2017)
A three-button mouse

Peter da Silva eloquently asked, "Where are all the three button [optical] mice?" I agree with his exasperated sentiment. To clarify, what we're both talking about is optical/laser mice with a full-size set of three buttons on top, so that there's a "real" middle mouse button instead of just a scrollwheel which you can awkwardly press down on.

The reason we both care is that on systems running Xorg/X11 (so: most folks using Linux or other *nix/BSD systems), the middle mouse button pastes your selection buffer by default. So, it's a "paste" shortcut. It's quite easy to get used to, and was frustrating when scrollwheels started taking over that real estate.

It's not that I don't like scrollwheels -- my ideal mouse would actually be the Perfit Optical, which has the scrollwheel on the thumb, but leaves the middle mouse button alone. Alas, those are far more expensive than I'm willing to spend on a device that'll wear out as frequently as mice do.

HOWEVER, it appears that all is not totally lost:

That may be the only "real" three-button mouse available at the moment! Unfortunately, it doesn't have a scrollwheel, and seems to have been removed from HP's website as of July 2017, so I suspect that these are not long for this world.

I actually do have to admit that for the past few years I've finally given up and started using a modern mouse with a middle scrollwheel, and I've just retrained myself to use the thumb button as my "middle" button, which works well enough. In an ideal world I'd still like that middle button back, though!