Reiser is a new filesystem for Linux developed by Hans Reiser at a company he owns. The filesystem is very advanced supporting packing tail's, indexing directories so large directories are stored efficiently, and also supports Journalling.

Hans Reiser believes that file systems should be good enough that you don't need to use a dedicated database to store data.

A lot of people don't trust Reiser yet, although it appears to be very high quality, reports of Reiser having file system corruption in the past have left many with the feeling that they will wait a bit longer before committing anything mission critical to Reiser.