I don't know if there's any software out there becasue I think it's controlled by a core part of the OS or even the PRAM or whatever. Not sure I'd want to try modifying it either in case it overheated.
Tips for less fan operation:
1) Set Enger Saver preferences to Automatic or Reduced, not Highest (not sure if this applies the TiBooks though).
2) Allow more ventiliation and/or work in a cold environment
Not much help I'm afriad. My 12" PowerBook has at least two fan speeds. Does your PowerBook even have a slow speed? Maybe it just has an on and an off setting and therefore you are limited by your hardware? Generally, my fan only comes on quietly after long periods of activity with lots of programs open, or goes loud when dealing with intensive apps like GarageBand or Final Cut Express rendering.