Like Others said; Jam Station.There are tons of choices for the type of vibe you want to playover and they are catagorized by genre. Also, many will tell you the key it's in which is handly to choose if it's major or minor and location. It includes a little bit of notation so you can see the chord progression etc. They aren't super long by the do somewhat loop (you'll hear a very breif stop and then will start from the beginning).

I wished I'd had a jam station way (way) back when I started. I use it now at times when I'm playing/practicing. It's fun and all whipping through some cool licks but having your playing in context (like a piece of music) is very useful and keeps you honest.

If you've never jammed over music much before and are improvising, be prepared to hate a lot of what you're playing. Ok, hate is a strong word but you'll realize that it's important to work out licks and themes in your playing. When you start to feel that 'ugh, I don't like the way I sound', take a minute to stop and think in terms of how you apply the scales and possible themes and licks to the music you're playing over.

If you're a rock or metal fan, it's good to know that Randy Rhoads spent hours upon hours working on his leads on the Ozzy songs. The effortlessness you hear in those songs were not effortless. As you jam, something to keep in mind.