How to make something transparent, but not refractive?

To make a material transparent, but not refractive, change its Opacity level instead of its Refraction level.

Examples

1. Uniform transparency:

Material setup:

2. Procedural noise in Opacity color slot used to create "cutout" effect:

Material setup:

Note: the "Clip" option next to "Opacity Level" will clip the opacity map to 100% black and 100% white values only. Any smooth gradients (values between black and white) will be ignored. 

If you want to create glass material that does not refract light (like soap bubbles or light bulbs), see: What type of glass should I use?