Single Rider Lines are present at Test Track, Expedition Everest, Rock 'n' Roller Coaster, and Millennium Falcon: Smugglers Run.
Be forewarned, Rock 'n' Roller Coaster's single rider line is very slow moving, and is typically not quicker than the actual line. They don't even post a wait for it. The other two are very efficient and are great to use.
Space Mountain also offers a single rider line once you get to the last room of the standby line, where you notify a cast member that you're a party of one. This is great for skipping the line if it's shorter. Depending on how crowded, and the cast member, they may make you wait farther into the standby line than what would make it truly "express."
Other attractions often call for "parties of 1," so be aware and hold your hand up when asked for a slightly more expedited queue.