DNS leaks can be a huge problem that can make your VPN connection less anonymous. Historically DNSLeakTest has been kind enough to provide a robust website to check for leaks and ways to resolve them (solutions mirrored locally here). Earlier this year the site was hit by an outage that left users with no good alternatives except to blindly flush their DNS queue, etc and hope it worked. The site is back up, but alternatives are needed.
Use it! Or be exposed.