This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.
Typically, you want a low average thrust and a short delay for
a glider. For example, a B class model would probably do
better with a B4-2 than a B4-4 or a B6-2. The Apogee 10 mm BP
motors are ideal for small gliders. Be careful of motors with
large ignition spikes, like the A10-3T or C5-3, unless you
want to re-kit your model. Core burning motors, including most
composite motors are not usually suitable for gliders. The
Apogee end burning composites: C4, D3, E6, and F10, are ideal
The new Quest Micro Maxx motors have sparked the development of some very
small lightweight gliders. See:
JimZ even suggests using Micro Maxx motors on some WhiteWings models.