ReiserFS is a new FileSystem for Linux developed by HansReiser at a company he owns. He believes that FileSystems should be good enough that you don't need to use a dedicated database to store data. Rather than following the traditional Unix-like inode / bitmap based approach, ReiserFS stores all data including the metadata in a BTree. This leads to blazing fast performance especially when dealing with a slew of small files, a situation in which a traditional FileSystem' performance breaks down. It supports packing tails and journalling.
Unfortunately, the tree structure used is also the weak point of ReiserFS: if any of it gets corrupted, chances are that much more data will be affected than under traditional FileSystems. Rather than losing a single file to corruption of an inode, you may lose almost the entire contents of your disk if metadata close to the root of the BTree is affected. Fortunately, the likelihood of this happening due to bugs has been dramatically reduced in more recent version of the driver. Hardware failure caused corruption is still a serious problem, though.
Many people therefor don't want to trust ReiserFS with critical data. However, it should not be dismissed off hand. Its capabilities to handle very large directories and deal with huge numbers of tiny files very efficiently make it a prime choice for cases where performance matters and data is replacable - it is strongly recommended as the foundation for a Squid cache or a news server.