Stuffit requires MacFuse to be installed. MacFuse is a dead project but there are still developers and companies putting out unofficial releases to fix bugs and probably the most popular is Tuxtera 2.2 or 2.2.1 which are both Lion compatible.
A correction and a clarification:
1. Stuffit does NOT require MacFUSE. It can make use of MacFUSE to mount archives so they appear as drives, but this is an optional feature.
2. MacFUSE has been superseded by two projects, Fuse4X and OSXFUSE. OSXFUSE has a compatibility layer that allows it to be a drop-in replacement for MacFuse, so programs written to use MacFuse can work seamlessly with OSXFUSE.