WinISD is free and can be found at http://www.linearteam.dk/. Screenshot below.
There are a bunch of these simple speaker and subwoofer box simulation programs. They have their specific uses but it should be understood that there are a lot of things this type of simple calculator does not take into account. WinISD shows results based on 2 pi space - groundplane with no other boundaries. This program can simulate closed, vented, passive radiator, fourth and sixth order bandpass speaker enclosures. It calculates frequency response, phase, max power, max spl, group delay, spl at any given power level, excursion, impedance, and more. You can apply theoretical filters to the response, easily make adjustments to enclosure and port size, tuning, etc. I only use WinISD for two purposes though. First, when I want to view several different models at the same time overlaid on the same chart. Second, I use it to check vent air speed since some of the more complex calculators I use don't have this feature.
The reason I don't use WinISD exclusively for my sealed, ported and bandpass speaker needs is because it is really too simplistic to trust unconditionally. WinISD assumes an infinite sized baffle and therefore it does not show baffle step losses or baffle diffraction. Furthermore, it cannot predict standing waves inside the enclosure or the ports. Depending on a lot of things these cumulative shortcomings could be fairly benign but on the other hand they could add up to significant design errors.
Only the Pro Alpha Version is worth downloading, the other versions are basically useless. There are several sources of extra speaker driver parameter files that can be downloaded and added to your driver database. Around 2006 or so I heard some talk of a WinISD double bass reflex calculator seeing limited distribution but as far as I know it was not offered publicly.