Find an active community website(call it X) that is related to your product and post some messages in the boards offering a free trial to X members. For instance, if your product is for investors, post some messages on Fool.com or MoneyMakersGroup. If your product is for gamers, maybe something on the IGN boards. For hackers, this site would do. For car buffs, maybe CarForums.net.
If it's a B2B offering, find a related vertical and look for forums or industry websites for that vertical. Again, offer a free trial or some kind of benefit to entice these customers to sign up and keep them satisfied while you work out the bugs.
Generally there are very active and loyal members at these sites that are very interested in new tools that would appeal to their niche community.