Web fonts let you link any TrueType fonts into your web pages using CSS, freeing you from the old limited set of standard fonts. They are supported in all modern (well, future) browsers: Firefox 3.1+, Opera 10+, Safari 3.1+, and also IE4+.
Page download size is critical as it directly affects the performance experienced by users, and many TTF files are hundreds of kilobytes in size. This web service makes web fonts more efficient, stripping out the thousands of characters that you don't need and leaving only those you want, while preserving the high-quality rendering features that the fonts may include.
Warning: highly experimental. The generated fonts are
definitely probably buggy
and you shouldn't rely on them working reliably. Feedback and
bug reports would be appreciated.
The font-subsetting source code is available here.
Contact me on #whatwg on Freenode.