Anyone like Tower Defense games? If so I heartily recommend the Gemcraft series as it is the best lot that I've played (and I've played quite a few), you can try the free ones on armorgames (all of them are there I think).
Gemcraft is a mechanically deep and highly replayable take on the genre, I have put well over 300 hours into this particular title on the web, it is highly addictive if you like this sort of thing, I was supportive of the developer and armorgames so they gave me a free key to the steam version, isn't that nice of them? The price of admission is well worth it though, of course I do recommend you try the free version first to get a feel for it.