I thought Defense Grid was a fantastic game... and if you're interested in seeing a sequel to it, Kickstarter is where you need to be:
http://www.kickstarter.com/projects/...defense-grid-2
They're looking for $250,000 by 14th August to make a DG expansion pack, and are at $55,000 right now.
For a full sequel, $1m might be a bit optimistic but I hope they make it.