Depending upon the size of your tank, some catfish that spring to mind are...
Debauwi (Pareutropius buffei): Must be kept in 9+ groups to settle in, very timid otherwise, reach ~9cm. Gorgeous guys to watch in big groups, they school so tightly! The other week at Southern Aquatics in Cadnam, there were ~25 in a shop tank that were schooling a ~5" doughnut ring
"Giant Otos" (Hypoptopoma sp.): Reach ~7.5cm, again socialable, something like 2 per 10 imperial gallons. Will help keep any algae under control
Chaetostoma catfish: Can reach upto 15cm, depending on species (still very unknown group of fish). Needs good water quality and excellent water currents with associated high oxygen concentration. I'm still trying to find out whether they need to be kept in pairs or trios or are fine alone (I'm seriously considering them).