A filesystem that concatenates files from an underlying
filesystem -- for instance, putting multipart VOB files from a DVD
back together. This could be extended to do various RAID-style things
with a file, encoded in the filename or passed to a control program
("mkcatfile linear /catfs/cat1 file1 file2 file3").
Piet Delport pointed out that you can do this at the block-device layer with many software RAID systems these days, which is definitely neater for the above usage.