How can I render glass in OpenGL?
It depends on the level of realism you want to achieve
To start, I advise you to use blending + reflection mapping.
For blending, see : http://nehe.gamedev.net/lesson.asp?index=02
For complex stuff, nvidia has some great demos with true reflection/refraction with fresnel coefficients. http://developer.nvidia.com/object/vertexprogram_refraction.html