So far we have experience with two motors: the OSEPP 25mm motor that comes with the OSEPP Tank Kit, and the micro gear motors (made by Pololu?). The latter are available in a variety of gear ratios. Both have a Hall Effect motor encoder kit available.
The choice of motors will depend entirely on the scale of the robots you might build. If you're fairly serious about robotics (i.e., you don't just want a remote-controlled toy), you'll probably want to choose motors that have Hall effect motor encoders either already attached or as an option. This is to permit you to precisely measure how far each motor has travelled, its velocity, and likewise perform odometry. This is absolutely necessary for autonomous navigation: if the robot can't travel in a straight line or tell where it is in 2D space it can't navigate, it just runs around avoiding obstacles (hopefully).
On the smaller end of high quality motors with encoders you can find lots on the Pololu website as "micro metal gearmotors" (AKA N20), with encoders. These come in a variety of power ratings and a large number of gear ratios. Pololu also sell a variety of larger motors, as well as motor drivers and motor controllers.
If you're looking at larger robots you can find a variety of motors with encoders on the ServoCity (Imperial measurement) and goBILDA (metric) web sites. These come in 22mm, 25mm, on up to larger motors. Of course, the larger the motor the higher torque, but also the often significantly higher power requirements (and lower battery life, therefore requiring larger and heavier and more expensive batteries).
If you're planning on having your robot operate outdoors or on uneven terrain, you'll want ball bearings on the output shafts. Plain bearings won't last long when bouncing over rocks.
So the first thing really to do is determine the scale of your first robot, then select a motor suitable for that scale.
These tiny brushed DC gearmotors are available in a wide range of gear ratios—from 5:1 up to 1000:1—and with five different motors: high-power 6 V and 12 V motors with long-life carbon brushes (HPCB), and high-power (HP), medium power (MP), and low power (LP) 6 V motors with shorter-life precious metal brushes. The 6 V and 12 V HPCB motors offer the same performance at their respective nominal voltages, just with the 12 V motor drawing half the current of the 6 V motor. The 6 V HPCB and 6 V HP motors are identical except for their brushes, which only affect the lifetime of the motor.
This is the motor that comes with the OSEPP Tank Kit. You can buy motor kits that come with an anodised aluminium wheel, a silicon tire. A Hall Effect motor encoder kit is available separately.
RobotShop sell a MiniQ Motor Wheel Set w/ Encoder that includes two micro gear motors, wheels, white plastic hubs, and a motor encoder pair — with the encoders integrated into the motor mount — for US$36. They use a 50:1 gear ratio rather than 298:1 (perhaps that's better?). This uses an optical encoder rather than a Hall Effect encoder, using the white teeth of the inner wheel hub for the optical interrupter, 48 pulses per revolution.