The world of web fonts has traditionally been limited to Times, Arial and Courier (and various variants thereof like Helvetica which of course came first). Other fonts can be used reasonably safely if you’re careful, or not too picky about certain target platforms displaying exactly the same font.
CSS has a rule (@font-face) which allows you to specify whatever typeface you want by making it download automatically to your users. Support isn’t universal, but improving with some clever CSS font hacks and so on.
I’ve always been keen to find nice readable fonts on the web, and I love projects like Readability too, which cleans up web pages of their clutter and puts the words you want to read (and not the flashing adverts, links, navigation, etc) into a much easier-on-the-eye and relaxing format.
Somebody recently tweeted a link to a free web font showcase which had chosen fonts specifically for on-screen reading. That led me over to Font Squirrel, where I picked up Charis SIL Regular and set it to be the font for the body of my articles on this blog. Previously I used Palatino (you may see Book Antiqua, Georgia or your standard serif font thanks to CSS font precedence rules), which is still visible in the sidebar – see the difference? Have an opinion? Let me know.