badsector implements the algorithm described in http://smartmontools.sourceforge.net/BadBlockHowTo.txt for the ext[2-4] partitions. The smartmontools can report problematic bad sectors in LBA addressing format. This program relocates bad LBA sectors, provided they are unused.
MUSE is an implementation of a "multiple mounts merging" filesystem. It's something like unionfs, but it allows writing to all the underlying filesystems at the same time. It is suitable for convenient unification of large disk arrays when fault-tolerance is not the top priority.