Clarendon is another great serifed font (and, for those interested, is used by a certain antipodean lager), and Web2.0 social web app site Ning uses it, complemented with a nice shade of green.

American Typewriter is like Courier on steroids - More stylized, prettier - and great for that faux-typewriter effect. 43Things uses it well, and is a pleasant break from san serif fonts.

Popular Web2 blog TechCrunch uses the perennially popular sans serif Frutiger. Simple, universally effective and clean, Frutiger is a mainstay of many graphic designers type palette, although perhaps not to the extent of Helvetica.

Helvetica is possibly the most popular san serif font in use today. Developed almost 50 years ago in 1957, it has seen use from signs to print, and now on new media and websites. 9rules knows that if you want a nice, clean, san-serif font, sometimes only Helvetica will do.