Agree with those above. For a NEW gun, you are looking at the Ruger SR9. You might also find a dealer with MP446 Vikings under $500 and they aren't bad.
If used, look for an older Glock 17, but even used they tend to be more in the $600 range.
Other options for used is an older Beretta 92F, Walther P1, post-war P38, Iraqui M1951, etc.
For between $500 and 600, I sometimes see Steyr M9A1's or S&W M&P9's in the EE, and they are great if you can find them in that range. On the other extreme, there's a guy now who wants well over $1000 for a Steyr M9A1, but lots of stuff there is overpriced chaff you need to look past when shopping.
EDIT: Although a lot of guys will steer you to a 1911 platform, I'm not a fan of them in 9mm. I do love the 1911 platform in .45ACP, but it's a big heavy gun for a small round like the 9 and the modern wonder-9 tupperware pistols will be infinitely more reliable than most any 1911 under $500.