I would create a post in several forums that I know of -
Essentially any game related forum. It's not easy 'getting it out there.' It's quite time consuming.
Aside from that, I would get active on http://www.moddb.com. There is a ton of activity and you can post your game, mod or add-on.
If you sign up on gamedev.net and pay a yearly fee one of the perks is listing your projects in their showcase - http://www.gamedev.net/community/gds/.
If you're working on something really really slick, you might want to think about writing in to some of these websites and see if they would write a post / review your game. That could help quite a bit.