As a designer, my math is kind of crappy. I don't like to fill my head with figures when I'm working on making a site look good.
Imagine having only 4 fonts in your operating system. Silly, is it? But almost by accident that's what happened to the web. The best web fonts were Verdana, Tahoma and Georgia. What is special about these fonts is that they are designed to look particularly good on screen rather than in print.
Unfortunately for designers, working with three or four fonts is no fun. Even for end users, we are all getting tired of looking at the same typefaces. But until now there's been no solid working solution for using alternate typefaces. Those that did exist either did not work properly across all important browsers or they slowed down the display of your website.
First tools were moving into wrong directions. They were either transforming text into images or into Flash. Which makes the text basicaly unusable. Nice, but still unusable. Now the situation has improved. There are online tools, which still render text, but the output is text again. To list a few of them: