I've used this name when I was a kid: Thepoopometer.
"poopometer" stands for a thermometer that measures the density of any players poop.
I was a kid, give me a break.
I have seen:
Cream3142 (Tip: 3.142 is something in mathematics)
In case you're wondering, that's Pi. The funniest name I've seen is actually Team Fortress 2 Bot name (totally not a bot). Oh, and there was one player on TF2 who's username was "a stupid person". No joke.
Not sure if this counts, but on Battle Nations (it's a mobile game), I saw someone who's account name was "GeneralisedChaos". And on Please Stay Calm (another mobile game), I saw a player who's name was "General Disarray".