If what you are looking for is more open ended, it will be hard to find. If you are looking for a short term experience it really depends on what you want to accomplish and where. The sailing sites that have already been put forth are but a few if you do a search on sailboat charters, ASA, Offshore, etc..
One option that I find appealing is at
www.mahina.com . They have a schedule of their cruising plans and berth availability now through 2010. You can do anything from a week in the Panama Canal for $2000 to a month from Fiji to NZ for $6000. You learn and perform as crew as you go. They have been doing this for over 15 years, and seem very successful.