With the exception of Apple's now-defunct single-button mouse, input devices don't get much simpler than the Kensington Ci73 Wired Mouse. It has two buttons and a scroll wheel. That's it, folks--you can go home now.
Unfortunately, the Ci73 can't take advantage of Kensington's excellent MouseWorks software, which is designed for the company's more capable multi-button mice. Instead, you customize the Ci73 through Mac OS X's Keyboard & Mouse System Preferences pane--though "customise" is an overstatement. You can switch the Ci73's right-click/left-click behaviour and adjust its tracking, scrolling, and double-clicking speed, but that's all. You can't, for example, assign an alternate behaviour to the scroll-wheel button; clicking it is hardwired by Mac OS X to activate Exposé, which is certainly better than nothing.
The Ci73's USB cable is a full 1.5 metres long, which is more than ample for reaching the USB port on the left hand side of your MacBook, should you be a right-handed mouser; it may even stretch all the way to the back of your Mac Pro. Unfortunately, if you use a USB port on your keyboard, the cord is also long enough to get in your way.
Kensington Ci73 Wired mouse
Although it's a featherweight at 90 grams, the Ci73 carries much of its weight in its rear, giving it a tendency to pivot a bit when moved laterally. Also annoying is its small, somewhat tough-to-roll scroll wheel--my scrolling finger would sometime slide across it if I didn't push down hard enough.
Quibbles aside, the Ci73's tracking capabilities are quite good; I had no problem whatsoever doing minute pixel-by-pixel work in Photoshop and Dreamweaver. I also enjoyed the comfortable ribbed-rubber bumper enclosing the outside of the Ci73's shiny black plastic body. Ergonomically, I found its low profile to be a bit too low for my taste, but my hands are unusually large; if yours are less Brobdignian, you may find the Ci73 to be perfectly comfortable.
Macworld's buying advice
The Kensington Ci73 Wired Mouse is as basic a mouse as you can find these days. It works and it's inexpensive. If that's all you need, then you will be happy.