Unity has a bug reporting system that pops up on a crash. But if I encounter a bug that doesn't crash Unity, how do I report that?
In Windows, in your start menu, under the Unity folder, there is an app "Report a problem with Unity". You can run it from there and submit the bug.
On the Mac, you can just hit Command-Space and begin typing "Unity Bug Reporter". For me, I rarely have to actually type more than "bu" or just "b", but that will depend on what you've been using Spotlight for recently. I'll assume on Windows Vista or 7 that you can type the same kind of thing; you'll just have to press the Windows key first.