This gear motor is very comparable to hobby servos, and has a 90 degree output shaft.

Specs

It offers 50 in*oz of torque, rotating 360 degrees every 1.6 seconds (38 rpm - just a hair slower than a servo), at 5V, drawing 600mA at stall (free running at 52mA).

Features a 7mm double-flat output shaft (avoid using the "D" output - it's not meant to take rotational load), and a built-in clutch (limiting at 60 in*oz, but easily locked), and built-in mounting screw holes.

Overall dimensions are 70mm (2.76") long x 22.5mm (0.886") x 37mm (1.46") including output shafts, while only weighing 37grams or 1.31 ounces.

Check the DC motor how-to to get started.