Font Squirrel’s @Font‑Face Generator ↦

You give it some fonts (licensed for use online, of course) and it gives you the files and CSS markup you need to embed those fonts in your website. It handles format conversion (including Internet Explorer’s EOT format), hint modification (for better rendering on Windows) and can even filter out glyphs you don’t need (for faster downloading). If you want to embed an open source font, check out their gallery of pre-made font kits so you can save their server some processing time.