Some faces I like:

serif
Garamond
Linux Libertine
CMU Serif
sans serif
Lucida Sans Unicode
Go
Linux Biolinum
monospace
UNI-VGA
Spleen
Go Mono
CMU Typewriter Text

Note that it’s a stupid idea to declare specific type faces for websites. If you need a fixed-width face, use monospace. If you don’t, don’t use any font-family declaration.

Tell your browser whether you want faces with or without serifs as default. Tell your fontconfig what faces you want as defaults for serif, sans-serif, and monospace. (And note that Firefox measures the default font-size in CSS pixels, not in points.)

Yet to check out:

Kurinto
good-looking family, including metric-compatible faces for Arial & Co.
Iosevka
nice parameterized fixed-width face

Sigrid dear has converted Kurinto and Iosevka to Plan 9.

I really should rotate my type faces. Maybe an XSLT script for randomizing the fonts.conf?