Many proprietary fonts are encumbered by copyright restrictions, so I always keep an eye open for sources of free fonts. Even so, just because someone announces a font is free on a website, if you plan to use a font in anything you publish, it is wise to first check the font license. I will continue adding to this list as I discover new font resources.
And of course, I welcome any additions you share with me 🙂
DaFont is where I found the beautiful public domain “Rebel Caps” font I selected for the title type face of 
SIL International developed the OFL (Open Font License) and provides several excellent fonts, including Gentium and Charis.
Linux Libertine's "Lively Alphabet"
The Fell Types offers an interesting historic view
The League of Moveable Type has an interesting blog
Floodfonts type foundry: Free Fonts
Vistawide Free foreign language fonts
Brushez Arabic and Russian fonts
and of course there is Google Fonts, which are designed to make it possible to use free fonts on websites. I myself have a problem trusting any corporate entity to continue supporting any software forever, so I prefer to download any fonts I use there. Google discourages such font downloading, since it makes it more difficult for Google to track the use we make of such fonts.
Just added: the collection of all the free fonts in the Debian Operating System can be found at http://fonts.debian.net/
(thanks to Patrick Niedzielski for the tip!)
