You may not search through many newsgroups at once for instance or search for files in a large list of groups. Usenet indexing sites try to aid users by offering a way to search the contents of many newsgroups at once. They index new posts that appear in the observed groups and offer web interfaces that users can use to search for files.

Most of these indexing sites support the nzb format which has all the information about the files that you want to download. If your newsreader supports nzb you simply double-click the file and all information about the files are automatically added to your download queue.

I know of some users who only work with nzb files and do not download headers anymore. We also have to distinguish between public, private and pay indexing services. While I wont talk about the private services at all I would like to point out that they are usually forums where users share their finds and uploads.

A short explanation for the terms “automatic content” and “user content”. Automatic content means that all new posts are indexed and can be searched on the site while user content means that users report new files and only those are added to the database.

Alt.Binaries.Nl (free, automatic content) – indexes more than 2700 popular newsgroups for 45 days. NZB’s can be generated as well. Great but Binsearch is better.
Binsearch.info (free, automatic content) – probably the best service. Keeps records of more than 6500 groups for 120 days. Advanced search options and the ability to download everything that you like as a nzb. My recommendation.
Newzbin.com (down) (paysite, user content) – one of the oldest indexing sites on the Internet. You may take a look at the releases but you can’t see the filenames nor download nzb’s without paying for the service.
Ngindex (paysite, automatic content) – Ngindex has a great interface, probably the best with many information about the newsgroups that they index. Offers nfo previews and more but is unfortunately a paysite.
Nzb.to (invite only) (free after registration, user content – a user driven website in German with an excellent active community. Lots of nzb files are posted everyday. Mainly interested for German speaking folks.
Nzbrus (free after registration, unknown) – I can’t really say much about the site because I did not register at it. Just wanted to make sure it is mentioned here.
Yabse (free, automatic content) – indexing posts for roughly 100 days. Allows the generation of nzb files.

There is also a program called NZB Leecher which is able to search several of the mentioned websites and create nzb’s automatically from the results.