Scoble's interview with Bill Hill reminded me that font choices are another weapon in the ongoing battle against information overload. Of course, any conversation about fonts is almost as likely to escalate into an all-out religious war as is a conversation about, say, code indentation practices*. So I'll start out with relatively bland points, and then move on to the more controversial stuff. Here goes:
  1. You need ClearType. No, really, you do - unless you're one of the small minority who are too color-sensitive. And you probably need to tune it.
  2. It follows from #1 that you need LCD screens - it's the 21st century, and making little bits of phosphor glow by putting them on the inside of a giant vacuum tube and zapping them with electron beams just doesn't cut it anymore. Use your laptop as a second screen if you have to.
  3. Microsoft's "core fonts for the web" have been designed specifically for readability on computer screens, and you'd be crazy not to try them out first.
  4. If you've made it this far, you need a way to compare fonts easily and quickly at multiple point sizes. I like this page from Jeff Howard.
  5. Sans serif fonts are great for absorbing small amounts of information quickly - so use one for email. As Omar Shahine points out, the mainstream choices are Verdana, Tahoma, and Arial, which are basically three narrowing variants on a theme.
  6. Serif fonts are better for reading large amounts of information, because the serifs help the eye move horizontally along a line. They're probably not a good idea for your typical web page, but try a serif font if you're writing a lot of text. Just don't use Times New Roman on a screen; Georgia should be your default.
  7. Consider using a "personal" font for email, especially if you're in a small team. If Fred uses Verdana and Jim uses Tahoma and I use Trebuchet, it's a lot easier to instantly get context in any long email thread.
  8. 11 point text is easiest to read. 10 and 12 point text are ok. Don't use anything else for extended sections of text.
  9. Any modern IDE will let you code in whatever font you like, be it proportional or monospaced. Here the "weight" of characters such as ()<>{} becomes critical. They are often afterthoughts in font design, and can be screwed up in either direction - too heavy and they dominate your identifiers, too light and your structure is hard to parse. As with Peter Golde, I like Verdana.
  10. If you still insist on monospaced for coding (or, even worse, if you're stuck with an IDE that insists on it), at least use a sans serif font. And since Tobias Jung put all that effort into hunting down ProFont, you could give it a go. But please, not at 8 point. It's far better to get a second screen…

*Three-space tab stops. You know it makes sense.

[Update: I've added another post on ClearType]