Mac OS X can read (but not necessarily write):
- HFS+ (Mac OS X default format, in plain, journaled, case-sensitive, or journaled/case-sensitive varieties)
- NTFS (but not write, by default)
- FAT (32 and 64 as seen in Christian Correa's comment)
I would expect it can read but I haven't tested it:
- HFS (the older Mac OS file system [Hierarchical File System])
- UFS (Unix File System; older versions of OS X could format and be installed on UFS partitions)
I know for a fact that OS X will not read:
- ReiserFS
- ext* (any of the various versions of the ext Linux file system)
- just about anything else
Check out the Mac OS X column at the incredibly complete Wikipedia article comparing file systems.