The Gigantic Servo Chart
Most servo manufacturers make their servos look better by advertising the torque and speed ratings at 6 volts or more. This makes the servos typically stronger and move faster, so the servos seem better, but in doing this consumes more power from the battery. In the chart below, both ratings are listed (if available).
In the example above, the torque rating of 45 oz-in is at 4.8 volts and the 56 oz-in is at 6.0 volts. Similarly, the speed ratings of 0.20 seconds and 0.16 seconds are for 4.8v and 6.0v respectively. Here's a handy converter to convert between oz-in, kg-cm, and other units: Measurement Converter.
Also, for each different brand of servos shown, some of the extreme values (minimum and maximum for weight, torque, speed) are colored in blue text in the tables below. Some servos listed also have links to where you can get more information or how to order.