aMule won't be supporting Growl any time soon. aMule is a cross-platform program and can't really be customized to take advantage of the neat features of each platform it runs on.
That said, however, it's possible to cobble something together. In CVS and the coming 2.1.0, aMule will run a command of the user's choosing when a download completes. If Growl has any sort of command-line interface, you can use that to produce a Growl notification on download completion. I just checked, and Growl has an
AppleScript interface, and that can be invoked from the command line.
Also, aMule has the Online Signature feature which is really just a mechanism for external programs to check on the status of aMule by reading a simple text file. Somebody already used that to display aMule's upload and download speeds on the Mac OS X desktop, using another third-party tool. Sadly, I can't find the thread.
Check out the
Dashboard Widget somebody made for aMule, too.