Here is a simple, low-cost, high quality servo for all your mechatronic needs.
This servo is very similar in size and specifications to the Hitec HS-55. 
This little guy is able to take in 6 volts and deliver 20.8 oz-in. of maximum torque at 0.10 sec/60°

This small servo comes with a standard 3 pin power and control cable, and all hardware listed below.

Note: This servo is rated for 180° rotation, but in actuality it only hits ~160°.

Features:

• Voltage: 4.8-6.0 Volts
• Torque: 16.6/20.8 oz-in. (4.8/6.0V)
• Speed: 0.15/0.10 sec/60° (4.8/6.0V)
• Rotation: ~160°
• Single Top Ball Bearing
• Nylon Gears
• 3-Pole Ferrite Motor

Includes:

• 1x Micro Size Servo
• 1x Single Arm Micro Horn
• 1x Double Arm Micro Horn
• 1x Four Point Micro Horn
• 1x Circle Micro Horn
• 2x 2x8mm Phillips Screw

Dimension:

• 31.8 x 11.7 x 29mm
• Wire Length: 160mm
• Weight: 9g

Documents:

• Datasheet
• mbed Example