I recently surfed into a nice site called JamCenter.com where you find a “jam machine” – an online loop player that plays backing tracks to jam with. Simply choose a key in the left sidebar, scroll down (the loop player appears at the very bottom of the page), select one of the styles (rock, cool, metal,…), and the player will play a loop of a certain length. The chords are displayed so you know what is being played. The sound of the recordings is alright and the styles are often tasteful and fresh. Have fun!
If you ever need an online guitar tuner or a metronom, these can be found there as well.