http://www.droidarena.com
Really, it's more than just another addictive HTML game!
Players own one droid, which can be selected from several different chassis, each of which comes packed with standard weapons and an inherent advantage/disadvantage. You can mod your droid with custom weapons and armor and fight other people's droids for experience and credits.
The best way to get a good droid, of course, is to work on your base, which contains buildings that produce credits (CR), construction points (CP) and research points (RP). CP are required to construct more buildings and better weapons, and RP are necessary for research, which paves the way for more advanced buildings and weaponry.
Tips on leveling fast for the uninitiated:
1. Join arenas all the time. Nothing is going to happen to your bot if you get killed -- everything gets regenerated instantly when the arena is over.
2. Build a LOT of buildings. Fill up your base as quickly as possible. The more CR, CP and RP you have coming in, the faster you will level. Get as many Marketing Centers as quickly as you can.
3. Always be researching everything you have resources for. You will ultimately have to research everything, so there's no point in being selective.
4. Join a corporation. It pays to have allies.
5. Buy spies, but don't use them until you have a lot. If you fail a mission, a lot of people will retaliate and probably succeed.
6. Don't mod your bot until you get fairly high-level, and don't mod with anything that isn't maxed, unless you can get it dirt cheap from the market.
7. One word: DeathRay.