I sometimes sing in the shower but i tend to hum guitar parts instead (whatever song i am learning at the time) Its probably better that i tend not to sing as i really suck as a singer.

I do however sing at work almost constantly because my job is very boring and i'm often working with no-one nearby (fortunately the machinery drowns me out) although the other day i was singing "camarillo brillo" by Zappa when i suddenly realised someone was behind me giving me wierd looks. (not sure whether due to my voice or the lyrics)