UPDATE: This article is out of date and nzbX.co is no longer in operation. Click here for a current list of alternatives.
It seems like a lot of people have freaked out since NZBMatrix shut down. If you want something with the community aspect of NZBMatrix that you can sign up for right now you should check out nzbX.co. It’s brand new, and already has over 100,000 users. It has a robust API and currently can be used as a source in CouchPotato. It isn’t supported by SickBeard yet, but you can get a modified version of SickBeard at https://github.com/LemonadeDev/SickBeard that does support nzbX.co as a source.
Maybe you don’t care about the community aspect and just want a dependable index. If so, you should consider rolling your own. It’s not too hard to do with newznab. In fact, most of the indexes popping up in the wake of the NZBMatrix shutdown run on newznab.