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There are a variety available, so it depends on your distribution and your own choices. An old standby was ReiserFS, though it's rapidly become outdated, and its creator went to jail before Reiser4, its successor, got wrapped up. I'm assuming development on it somewhat continues, but I'm not sure if it's ever gotten a full release.
Also out there is the ext filesystem. It's been the traditional warhorse for a while, from the old days of ext and ext2, through ext3 and finally the current ext4 incarnation.
Some third-party ones from companies include jfs (from IBM) and xfs (originally created by Silicon Graphics). There are also plenty of experimental filesystems out there, including one that's been gaining some mindshare called btrfs, from Oracle.
Finally, Linux itself supports a variety of other filesystems that aren't generally considered Linux/UNIX filesystems, including FAT32, which was once the standard for Windows.
EXT2, EXT3, ReinserFS y XFS.
EXT3 is better than EXT2 and ReinserFS y XFS is better than EXT3.