I learned about Cacoo from @NikPeachey and immediately started using it make diagrams. It’s extremely easy to use. My favorite feature is probably the ability to snap-to. I am pretty lazy when it comes to lining up parts of a diagram and tend to eyeball it. Now, I don’t have to!
One of Cacoo’s selling points is that it allows realtime collaboration on diagrams – but I haven’t tried that out yet.