DISCLAIMER: This game looks lame as fuck, but if you can get past that it's actually pretty awesome.
You've probably heard of it; it's a simple capture-the-flag flash game. The rules are what you expect: You take the flag from their base, you take it to yours. You tag the flag carrier, they lose the flag and it respawns at your base. You can't cap while they also have the flag.
Now, you might be asking, "But Ben, it's just a bunch of balls moving around on a lame looking map, what could be so fun about it?". For starters, there are elements such as boosts, spikes, bombs, and power-ups scattered around the map, all of which can help you mess with the other team and/or actually capture the flag.
Movement is heavily based on momentum, and you can use your understanding of your own ball's movement to pull some crazy jukes/blocks on the other team. There's actually a high skill cap, and it's a lot of fun when you figure out the basics.
If it interests you at all, there is actually a competitive community, but I don't know much about it and personally I don't want to get involved with that. More info on the subreddit, /r/tagpro.
Let me know if you end up playing. I'll totally be down to play in whatever free time I have.