Recs.
Updated
Google Fonts makes it quick and easy for everyone to use web fonts, including professional designers and developers.
The fonts can also be downloaded in .TTF format from GitHub Apache License and GitHub Open Font License.
Specs
Pros
Pro Live preview
You can live preview fonts in multiple ways - you can view a single word, a sentence or a paragraph with text and font-size of your choice. There's also a poster view that tries putting more fonts on screen at a time by showing only their name without the option of entering your own text, and is the only view option that allows inverting background and text colors.
Pro Easy to filter through fonts
Within categories such as serif, sans serif, monospace, etc, you can adjust desired thickness, slant, width and script to filter the collection of fonts. Additionally, you can also set how you wish to order the fonts (by popularity, alphabet, date added, etc).
Pro Great for web designers
This repository was designed specifically for web designers. As such it shows the impact of the font choice on page load time, allows selecting only the font styles and sets you need to reduce overhead, and gives multiple methods of integrating the fonts into your website.