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.