The certain members of the spudding community have written computer programs pertaining to spudguns:
Accuracy of the predictions is typically within 5-10% of the actual value, although there has not been much testing of very long where the (unaccounted for) flow resistance due to barrel length is significant, or very high pressure launchers where the non-modeled liquefaction of the propeller gas(ses) may take place.
Standing for Compressed Air Launcher Model this is a Pneumatic Launcher simulator program by AKB of the SGTC. Since it is written in the Python programming language you can read the code and see how it works. It is not graphical, but produces tables of pressure, velocity, height, etc.
CALM can be downloaded at http://www.qsl.net/wb6zqz/calm.html .
A newer simulation of a pneumatic gun written by btrettel. Like CALM, it is written in Python, is open source, and is not graphical. While it is not as advanced as GGDT, BAGS is generally within 5% of GGDT for similar inputs and subsonic speeds.
Changing options is done through editing the Python program at the moment.
BAGS was available at http://trettel.org/bags/ in the past.
EVBEC stands for Energy & Volume Based Estimator for Combinations. Written by boilingleadbath, it is based on Latke's chamber:barrel test data.
Hosted on spudfiles, it is free to download and use.
A great spudding tool made by Boogieman. Click here to try it out
Spud Calculators galore. Requires Microsoft Excel. Download
UnitConverter is a handy online converter. You can convert metric pipe sizes to U.S. sizes, pressure, volume, energy and many other kinds of units. Put a shortcut in your web browser and you're never more than a second or two away from converting, for example, cubic inches to liters.
Convert is a downloadable windows application with capabilities similar to UnitConverter. Put a shortcut on your start bar and you're never more than a second or two away from converting, for example, cubic inches to liters.
Predicts ballistic trajectory accurately taking into account both atmospheric and ballistic variables. Download
Update in pipeline, with plans to release beta version shortly. Update 1/9/2010: Most likely abandoned, but still functional. JoannaArdway, the creator,last posted on the forum Sep 14 2007.
Predicts and simulates ballistic trajectory accurately taking into account atmospheric and ballistic variables. The program is designed for TI graphing calculators. 
Metering tool for propane, butane, and mapp gas. Download