i only know of a few that do the single riders. I believe screamin does, possibly soarin over california, i think indy and splash mountain still does theirs. other than that, i am not to sure. but you can always pick up a map at the parks and it will tell you which attractions have single riders.
At Disneyland, only Splash Mountain offered it. Indy used to, but due to changes in policy regarding the viewing of the safety film, they had to stop offering it.
At DCA, some rides offer it, but all the rides have the right to not offer it at any time, usually during slower periods.
California Screamin' is usually open, Mali-Boomer is only opened during peak times. Mulholland Madness is usually open unless the park is dead. GRR is only offered during peak periods. Soarin' usually offers it, but it is the one ride where you could end up waiting longer than getting a Fastpass to ride, due to the way the CM's group 9 rows of seats.
Every time I have used it for Soarin, I wait a long time and end up in the third row. So last time I was there I got a fast pass and then when I got up there I asked nicely if I could sit in row 1 in the middle. No problem, especially as I was alone. I also got on much faster