I bought Robot Wars and Project Earth recently in a second hand shop.

Robot Wars is basically like the TV show except more fun where you build armour plated, weapon weilding remote controlled cars to trash other "bots".
You enter competitions, win money and buy better bits for your bots.
You can also purchase pre-built robots such as Choas 2 or Panic Attack once you've unlocked them by completing competitions.

Project Earth is almost identical to Homeworld except there doesn't seem to be a limit on the amount of ships you can build, hence you can command much larger fleets.
On the downside the loading time for the missions and even returning to the menu screens is bloody diabolical and you may have to turn the detail and texture settings down to minimum as the game "chugs" quite badly on my PC when there's to much on the screen.