Tim at the Life in Student Ministry blog has thoughtfully offered up his top websites for use in youth ministry. Thought I'd jump on the bandwagon and offer a few of my own:
Pastor 2 Youth - This site is a wealth of free programmatic materials for those who work with youth. Once you sign up for free membership, you have access to tons of lesson plans, bible studies, powerpoint games, templates, and presentations, links, leadership tips, game ideas, and more.
The Source for Youth Ministry - Another excellent site for free resources that you can adapt to use in your own ministry. Catergories include games and icebreakers, team builders, event and activity ideas, movie clip discussions, and stupid skits, to name a few.
Creative Youth Ideas - Yet one more site full of free activities and ideas that are easily adapted, particularly when it's one of those weeks when there just doesn't seem to be enough time to get everything planned. I also like to steer leaders (both youth and adults) to this site (and the ones above) when they need inspiration for leading part of a youth group program.
Johnny Baker Blog: Worship Tricks - Johnny Baker, a blogger from London who is part of the alternative/emergent worship movement, shares a wealth of creative and inspiring ideas (he calls them "worship tricks") for making worship more interactive, challenging, and sensory. Some of his ideas may be more elaborate than you'd want to try with your youth, but they are adaptable and will likely inspire some ideas of your own.