I'm going to quote this from a few weeks ago when I analyzed the Marriott travel package value. For those worried about SPG transfers going away, Marriott points can be transferred to the following airlines:
United, Aeromexico, Air Canada*, Alaska Airlines, American Airlines, British Airways, Copa Airlines, Delta Air Lines, Frontier Airlines, GOL/Varig, Hawaiian Airlines, Iberia Airlines, Virgin Atlantic, Aeroflot, Air China, Air France/KLM, Alitalia, ANA, Asia Miles, Asiana Airlines, Avianca-TACA LifeMiles, China Eastern, Emirates, Etihad Airways, Japan Airlines, Jet Airways, Korean Air SKYPASS, LAN, Lufthansa Miles & More, Multiplus, Qantas, Qatar Airways, Saudia Airlines, Singapore Airlines, South African Airways Voyager, TAP Air Portugal, Turkish Airlines, Southwest and JetBlue via these travel packages. It takes a lot of points but the value can be pretty nice (see my quote below for analysis of the Southwest option - you'll want to focus on the lower portion of the comment).
EDIT: for some reason the link in there isn't working but Here is nights and flights:
http://www.marriott.com/rewards/usepoints/morepack.mi