SpaceSimCentral

This is something I came across today and haven't tried it out yet. Looks to be a tower defense style game and it is free to play on the web here: Orbital Guard have some fun with it and post your scores

I just played this game and I have to admit its the type of game that can keep you at it for hours without realizing ! Example: I played it for like half an hour at work and I totally forgot I had customers waiting for me