I typically sing something to myself if no one is around to make fun of me, and it varies. Sometimes it's a band I love, like Bad Religion, other times it's something from the 80s, or 90s alt-rock.
Usually it's something I just heard - sometimes it's good and happy, other times it's annoying as heck. Those song bugs get in your head and they're almost like hiccups.
Today I was humming Standing Still by Jewel.. and I was singing All Shook Up by Avila.
Right now, it's Animal Rights by Deadmau5
I make up songs, and sing my own remixes of songs i don't like
I usually hum whatever is stuck in my head and sometimes I hate the damn song lol