Ken's Telescope Calculator
This telescope calculator takes the objective diameter, f ratio, focal length, eyepiece apparent field, and eyepiece focal length as required to calculate the estimated objective performance, telescope magnification, magnification per inch, true field, and exit pupil size. It is a great way to compare the characteristics of different eyepieces. The calculator requires a JavaScript compatible browser.
http://www.klhess.com/telecalc.html - Details

Aberrator
The Aberrator is freeware that generates star-testing images to show the effects of aberrations(distortions) influencing the quality of a telescopes performance
http://aberrator.astronomy.net/ - Details