Chase Sapphire Preferred is a pretty nice card, as it also allows for transfers to Hyatt, Intercontinental Hotels & Marriott (and Amtrak)
If they added 1 or 2 more airlines (and keep transfer option to UA after merger /w CO), it could be one of the best cards out there, IMO. If they added Delta, it would mean all 3 alliances would have redemption possibilities.
Outside of the AMEX cards, its the only one I know that allows transfers to multiple partners.
As mentioned, it has an annual fee, waived for first year. And make sure you get the PREFERRED version, not the free version, as that does not allow transfer of points.