What's the best process guide software for a small team?

We're only six people, so we've managed without much documentation until now. The problem is everyone has become the expert in their own area. When someone's away, the rest of us spend ages trying to remember how they normally do things. I'd like to start documenting our everyday workflows before the company gets any bigger but I also don't want another complicated system we'll stop using after a month. What have other small teams found works well?