Germany also has a small base at Dulles International Airport here in DC (to support training units here in the US). They used to have F-4's and Tornado's at Holloman AFB, NM for training (I think they pulled the F-4's out, not sure about the Tornado's).
Singapore has several units in the US. An F-16 squadron, CH-47's and KC-135's. Singaporean CH-47's were among the forst, if not the first, foreign personnel to take part in Katrina relief efforts. They are apprently based in Texas.
Canada has a squadron, personnel only, at Tinker AFB flying US E-3 AWACS.
Singapore and Germany train here because of domestic airspace restrictions and Canada has a unit here because of their close association with NORAD.