There are a number of different products currently on the market designed specifically to easily clean your glass bong/pipe. Most of these products have a large mark up so the manufacturer and retailer (usually Headshops) can make a profit.
The standard bong cleaners fall into two categories: soak and shake cleaners. Shake bong cleaners required the cleaner to agitate (shake) the bong; these cleaners often have small granular chunks that rub against the inside of the bong helping scrap off THC resin.
Soak bong cleaners are designed to have the cleaning solutions left in the bong for a while. (more than 30 or 60 minutes) These cleaners absorb the THC resin and flush it down the drain. This is a longer process and not ideal if you are looking to use your bong soon.
Both of these styles of products work well to clean a bong, but there is a simpler solution. Using both rubbing alcohol (as concentrated as possible, 99% is ideal) and salt, will clean your bong very quickly and should cost less than a dollar per clean.
The rubbing alcohol dissolves the THC resin and can be left to soak if the bong, or part of it is very dirty, ex percolators often get really messy. The salt is added to the solution so it can be shaken and the salt will rub against the inside of the bong, scrapping the THC resin off.
The combination of rubbing alcohol and salt create a great mixture to quickly and affordable clean your glass bong and/or pipe. With most of us using our favourite glass piece (bong, pipe, rig, etc.) daily, keeping it clean can be a lot of work. Hopefully using rubbing alcohol and salt can save you some time and money!