So you're in a hotel room with your Mac laptop and your roommate's got a PC and there's one Ethernet jack. What do you do? You plug in your Mac and get to sharing the internet love, that's what you do! In System Preferences, go to Sharing and on the Internet tab, click the source (Ethernet) and the way the other computer will connect (for wifi, Airport). Then click the Start button.
This'll set up a wireless connection between your Mac and the PC, and the PC will be able to surf on it via your wired Mac. When I first enabled this in the hotel room here in Chicago, it told me I had "settings that might intefere" with the sharing. Turns out that "Personal Web Sharing" has to be enabled as well on the Services tab.
If you don't want everyone else in the hallway sucking down your bandwidth, click the "Airport Options" button to set a password on your ad-hoc network, before you hit the start button. Give your roomie the password and from there you're good to go.
Or you could just remember to bring a portable router. Either way.