Redweek.com. I've rented through them for the last 8 years and
have had great success. I've also been able to negotiate rates.

I used (at the recommendation of tons of people on the DIS boards) vacationstrategy.com to rent in a Wyndham property near Disney World. I think they can rent anywhere in the Wyndham timeshare network.

http://tug2.net/timeshare_advice/find_timeshare_rentals.html

Any suggestions on a reputable website
to use for renting someone's timeshare?