A friendly sans from the League.

A classy, free, quirky-futuristic sans-serif that allows @font-face embedding with a link back to the site.

Lovely modern sans-serif. I think this is the one Feltron uses all the time?

A gorgeous modern, light and clean open source font from a design class in Italy.

Nice sans serif in the style of Futura

...an overlay comparison

Exljbris has a quite a few nice free fonts (available for @font-face embedding, too!)

Hannah is a thin font with drastically variable-width characters and a hand-drawn quality. It would be fun for an avant garde or crafty piece. It reminds me a great deal of the opening credits of Dr. Strangelove.

The modernist sans-serif that is used in UK hospitals, train stations and airports. Has a very close aesthetic kinship with Helvetica.

Kottke's free pixel font