Software documentation

Was looking for resources for writing software documentation for a friend (and for myself as well). Figured the STC would be the most authoritative resource. On the STC Four Lakes, Michigan site, I found their book list to be enough, but figured there must be some guidelines for writing documentation on the web. Anyone know any good freely available resources /guidelines for writing software documentation that might be available on the web?

Kristjan pointed out one we blogged from Advogato a while back: How non-programmers use documentation.