I've been making them.
I put together a USB based module with 20 cues for about £100. Up to 4 modules can be used on one USB bus. The software was written in VB6 and allows for millisec control of the cues, along with playing an MP3 track as well. I use a 100m USB extender to get the modules out in the middle of the firing area.
The results were quite good for a homebrew system. See the link below (warning large file, and DivX encoded)
http://www.regiment..../welcomedub.aviThis shows the first outing of the system. I dubbed the music track on as the sound quality of the original was not good, but you can see what I was trying to achieve.
I'm currently working on V2 of the firing module which will feature a full TCP/IP stack and wireless operation. Current construction costs of this system is about £300 for modules up to 64 cues, with no limits on the amounts of modules being used. Perfect for larger shows where cue synchronisation is required over large distances where cables cannot be run (eg: Firing from boats/barges)