Usenet

In my experience, the invite-only ones are cleaner, index releases faster, and have less junk. They also typically have active support communities so you can talk to a real person if you need to. There are some open paid ones that are great (nzbgeek is pretty solid), but the invite-only ones index significantly higher quality content in many categories. Though to be honest, depending on what you’re looking for and how picky you are about quality, a couple of the standard paid ones would probably sufficient.

Some of the invite-only indexers do open their registration periodically, so I’d recommend keeping their registration page open and just refresh it every day if you can’t get hold of an invite. Drunken Slug, for example, opens their registrations to the public every so often.