Well, you've got lots of options when it comes to buying a mouse. First is how many buttons you want. Will a simple 2 button + scroll wheel mouse do it for you, or do you anticipate using your mouse for stuff (like games) that enjoy having multiple buttons? There are everything from $15 cheapos to $80+ 5-button monstrosities.
Second, how do you want to connect it to your computer? Is it fine if your mouse is wired, or do you want a wireless device? Most wireless mice use radio waves to transmit your movements; these come with their own receiver. Some use Bluetooth, which needs only one adaptor for all BT devices if your computer doesn't have it already.
Will this be a mouse simply for the desktop, or do you anticipate using it in mid-air? Most mice use an optical sensor or a trackball (though not as much anymore) to move, which require a flat surface to use them on. I have seen a few mice that use a gyroscope to triangulate your movements in mid-air; this is useful if your desk is too cluttered for free motion, or if you give lots of presentations.
Another thing to look for is ergonomics. Some mice are much more comfortable to hold than others. You should definitely compare mice in a store for this, see what kinds of shapes and sizes you like. Some are simply smooth and curved, others have grooves for your fingers, though these types tend to be uncomfortable for left-handed people.
The least important feature is looks. Some mouse are plain, boring white rodents and some are ultra-cool, sleek devices. A lot of the design factor is determined by the mouse's ergonomics.
There are also trackballs, have you considered getting a ball? I personally prefer the mouse, but you may like a ball better.
In any case, take all these factors into consideration when you go buying mice. Definitely go to your favorite electronics store (not necessarily an Apple retailer) and try different mice out. And, of course, make sure it has USB.
• 2.5 GHz MacBook Pro Core 2 Duo, 4 GB RAM, 320 GB hard drive, runs 10.6.8
• 1.6 GHz iMac G5, 1.5 GB RAM, 120 GB hard drive, runs 10.4.11 (out of commission)
• iPhone 4, 16 GB, iOS 4.3.5