I've been a Mac user since the 512, yet this took me by surprise. Suddenly the type on just about everything on my desktop seemed a bit darker and a little blurry. I was convinced that my monitor was going ... or worse, the computer was not sending it the correct voltages anymore. I was minutes from getting it ready for repair, then I decided to look around a bit.
Somehow I had hit a keystroke combination which barely engaged a zoom feature -- just about 1% or so, barely off of zero. I didn't even know I had a zoom feature! I went to the System Preferences » System » Universal Access, and turned Zoom off (how did it get turned on?). This immediately brought my monitor back to the razor-sharp images I had been looking at for years. Whew! A close call, based on a very simple thing that many of us forget is even there.
[robg adds: The keyboard combo is Command-Option-Equals, and I think Zoom is enabled by default.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070503111630678