That is a female Phidippus mystaceus Jumping Spider. They are known as the "moustached ones" for reasons that you can see in your avatar.
How do I know this:
A couple friends gave me one a few weeks back as she was about to be squished. Sure enough, as soon as I set her up in a real sweet enclosure, she produced an eggsac and is carefully tending it. If it is fertile (fingers crossed, I hope it is), I will be overrun with spiderlings. I will release most of them out in the pastures, but plan on keeping a few.
They are very fun to watch. Mine eats crickets. It is very violent. She jumps down on them from above. It is quite fascinating to watch.
She is very inquisitive, and watches everything that goes on in her cube. If I make a noise on the desk she will spin around and stare at me.
I just thought you would like to know!