I've been singing in the shower for as long as I can remember. Generally whatever comes to mind, no particular songs or artists, just anything that happens to pop into my head. Sometimes its one song, experimenting with octives and changing the key around for kicks, other times its multiple songs in one "session" forming a concert, if you will
I'm not really embarrassed about it, even if people overhear. I don't have a great voice by any means, but I can stay in tune so it doesn't sound so awful (or so I like to think), and in the end I enjoy it, so why stop? If anyone ever really complained I suppose I would, but not because some one makes fun of me for it.