Fonts

Libre

Google Fonts
Easily the most popular, and with good reason! All the fonts used on this site came from Google Fonts. Be warned that older versions of some browsers do not display some Google Fonts properly, so you'll want fallbacks for older browsers. The fonts on Google Fonts aren't as nice as some you'll find on Typekit or other foundries, but this is the best way to go for totally non-restrictive fonts.

Adobe Typekit
Typekit has hundreds of amazing fonts. Unfortunately, you have to create an account to use them, and there are only a few that don't require payment. However, the quality is worth giving them a mention here. Be sure to review the Typekit license, as it isn't a typical license and has some restrictions, but depending on what you're doing you can get quality fonts for your project without paying a dime.
Lost Type
Lost Type has some great fonts, but unfortunately they are only free for personal use; commercial use requires payment. If you aren't making money from your project, give this site a visit.

Open Font Library
OFL has hundreds of quality fonts in various languages. They support a wide variety of licenses, and while all of the licenses should allow you unrestricted us, I'm putting it in the Mixed category to make sure you know that you should check the font's license before using it, as even different open licenses could have different terms on what you can do.
Fontspace
At the time of writing, Fontspace has over 28,000 fonts, which makes finding something you like a little tricky. The licenses are all over the place as well, but there are some non-restrictive licenses.
Font Squirrel
A tightly curated collection of fonts that should all be free for personal and commercial use, but each font has different licenses that you should examine. The fonts here are top-notch, but some take more effort than others to use in different projects depending on the formats provided.
Font Fabric
FontFabric is a font foundry, meaning that all fonts on this site are created by FontFabric themselves. This is a perfect place to look for a non-typical font for a specific purpose. Most all fonts are free for personal and commercial use, but not all.