On some little, red, sporty car:
Z U NHVN
Hmmm...is he perhaps a Universalist?
On a somewhat beat-up black car:
<3 2 GTAIR
Sounds like a musician. Maybe.
On a silver Envoy, near my house:
LVNGOOD
I think I'm offended by that.
And my current favorite, spotted on a midnight blue Lexus:
SHDW CAT
*geeky glee*