The following hubs which sometimes come stock on new bicycles can also be purchased separately and installed on your favorite rim. I installed a Nexus 8-Speed on my Xootr 20" wheeled bike. It works very well so far.

1. SHIMANO NEXUS   Used on many comfort/street bicycles. I use the 8-speed redline premium on my bikes.
    
2. SRAM SPECTRO   (see S7, P5, T3)
    
3. ROHLOFF   Internal-gear hub!
    
4. STURMEY-ARCHER   They make and sell bicycle hubs and shifting systems.

Nexus Red Band Premium 8-speed Hub