Find Any File searches your local and network disks for files by name, creation or modification date, size or kind.
As there are other tools with a similar search operation, here are the special features unique to Find Any File:
Unlike Spotlight (i.e., the Finder's Find command), it does not access a pre-built database but searches the chosen volume directly. This allows you to find any file, even those inside packages and others excluded from Spotlight search. Hence it is great for finding system files, for example.
On the other hand, it may take a little longer than Spotlight, and it is only fast on HFS(+) volumes. But even on mounted network volumes of a Mac OS X server it can still be surprisingly fast.
Hence, this is not an entire replacement for Spotlight but it can come handy in certain, if not many, situations.