My girlfriend was doing this kinda thing a lot to me before I made any commitment to her. She'd comment about being single and going looking for men, talk about hot guys in front of me etc.
Basically, it was her way of letting me know she wasn't going to be there forever and kinda pressuring me into committing to her (Wyldfire wrote a tip on this in the tips section, which stuck in my head during this time).
I'm assuming you haven't made your move with this girl, right? Well, unless you are in a relationship you can't justify getting annoyed. Afterall, it's not like she's cheating on you and this is what she's trying to express.
The way I handled it was to play it like I was cool with it and call her bluff. I even offered to pass on her number to one of the guys she was checking out. Then she said, "err, he's not that hot really".
If you like this girl, make your move before she gives up on you and hooks up with some other guy.
Generally, if a girl seems to be making an effort to make you jealous it's a positive sign, but if you lose your cool and she can see it working, it will become her way of controlling you.