Without knowing what you already own, I would check out the Kel-Tecs. Several different frame colors and a choice between single stack mag PF-9 and the double stack mag PF-11, both in 9mm. You can pick up one, an extra mag, and maybe a box of "cartridges" for $500.
A tristar or EAA CZ clone (or possibly a used CZ but you probably won't see one)
A used S&W .38 revolver
A sig 2022
A used Glock of pretty much any flavor
A Ruger security six
A ruger P95
A Ruger SR45
Many many .22lr pistols
You might see a used ruger blackhawk in that range too. I saw a decent (70% finish but in good working order) blackhawk for $350 at. Gun show last December.
The list goes on and on...
$500 will buy you some very good handguns actually, you just have to look a bit