Today I've seen a reference to NILFS, a log-structured filesystem for Linux. It's interesting how they've put the most important "feature" at the bottom of the "Current status" page. Looking at the page, the first important thing one notices is that the work on garbage collector is ongoing. Doesn't sound well. At the bottom of the page, they conclude under known bugs with "The system hangs on a disk full condition." How nice :)
On a more serious note, I think that it's great that someone is working on alternative filesystems. NILFS can, for example, support "time travel".