FindNZB is a great free nzb indexer for Usenet newsgroups. The site offers a free search and download of NZB files. The service is public and open for everyone.
Newznab Plus Alternatives
Newznab Plus is described as 'Usenet indexing application, that makes building a usenet community easy. This the commercial version' and is an app in the news & books category. There are more than 10 alternatives to Newznab Plus for Web-based, Self-Hosted, Windows, Linux and Mac. The best Newznab Plus alternative is Binsearch. It's not free, so if you're looking for a free alternative, you could try Binsearch or NZBClub. Other great apps like Newznab Plus are NZBScout, altHUB, Spotweb and nZEDb.