Ellen Spertus on Stack Overflow’s blog:
While there are many resources to help programmers write better code—such as books and static analyzers—there are few for writing better comments. While it’s easy to measure the quantity of comments in a program, it’s hard to measure the quality, and the two are not necessarily correlated. A bad comment is worse than no comment at all. Here are some rules to help you achieve a happy medium.
I like rule #6 (provide links to the original source of copied code) and rule #9 (use comments to mark incomplete implementations) in particular.