The Solarbotics GM12a 100:1 mini metal gear motor is a compact (1.14" x 0.39" x 0.47") brushed DC motor with metal gears, very similar to the Sanyo NA2S motor. The major difference is that these Sanyo motor look-a-likes have brass front faceplates (not steel ones), and cost less.
Documentation and other information
Comparison of Solarbotics Gearmotors (GM Series)
Table comparing specifications for the Solarbotics GM motors that we carry.