What car rental companies have on-site lots at PDX as of Dec., 2012?
Avis, Dollar, Enterprise, Hertz & National are all on-site, according to: www.portofportland.com/PDX_Grnd_Trnsprtn.aspx
(Advantage, Alamo, Budget & Thrifty are off-site)
There are free shuttles to most if not all "off-site" locations. You can walk to the on-site locations.