Amtrak --> OnePass
5,000 Guest Rewards Points equals 5,000 OnePass miles. You can initiate this online by clicking here:
http://www.amtrakguestrewards.com/
(Just click on the Partner Rewards link. Make sure that you login first)

Marriott Rewards --> OnePass
Look here: http://www.marriottrewards.com/awards/airrewards.asp#6

Starwood --> Onepass
Look here: http://www.starwood.com/preferredgue..._checkRoc.html
(Note: For every 20,000 points you transfer, they give you 5,000 bonus points free)