Ask Slashdot has a great interview with Scott Trappe. One of the main things he emphasizes is pair programming and code inspections and I couldn't agree more.

If you pair an expert programmer with a novice programmer, obviously the novice will learn more. But the expert will learn as well because the novice will undoubtedly ask a lot of "why" questions, reinforcing what the expert knows.

As well, it provides code inspections while you write -- assuming the pair isn't collectively out to write bad code. The idea is that two heads are less lazy than one.

Pair programming, however, is usually only something that can work when developers are close to each other. Open source projects are somewhat immune to this because they allow everyone to see the code, including users. Thus open source programmers usually write good code because they don't want to be known as bad coders. Their pair is the little angel on their shoulder. :) Commercial developers are generally anonymous and don't have to worry about bad code directly affecting their reputation -- they still get paid either way.