Remix Fight is an online community that has bi-monthly remix competitions. Download a track in the form of a bunch of loops, remix it and submit your remix for judging by the community.
This is a great source of new tracks to practise your mixing skillz on. For best results, don’t listen to anybody else’s mix or ideas until you’ve finished your own. Then listen to all the other entries and think about how they approached their mix.
From the website:
How do I enter a competition?
To enter, visit the upcoming fights page (http://www.remixfight.org/mt-archive/cat_loops.shtml) and download the loops provided there, make a remix using whatever means you have at your disposal (see remixing resources if you have none), and then e-mail an mp3 of it to firstname.lastname@example.org with your artist name, artist website (if you’ve got one), mix title, and a short blurb about your mix.