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.

If your problem solves someone's need chances are they will find you. If posting a link on one or more sites frequented by your target audience doesn't generate a single beta tester you might want to re-evaluate what you're building.

For example I was a beta tester for GitHub and converted to a paying customer after the beta period. Someone must have put an announcement here about the site and since it solved my need for git hosting I immediately signed up.

Write a blog and publish the articles on social networking sites. You'll get beta signups naturally from that.