Internal-gear hubs offer the advantage of less maintenance compare to a standard derailleur based drivetrain, great for wet weather riding.  You can change gear when at a stop, a plus in traffic.  The disadvantage is the extra weight and increase in drivetrain friction.

Shimano Nexus (3/7/8)
SRAM/Sachs (3/9)
Sturmey Archer (3/5/7/8)
Rohloff (14)
Nuvinci (CVT)

Check out Sheldon Brown’s page on internal-gear hubs.