The one design i found for sale in the UK seemed interesting, the coolest part was that it was an easily reuseable pyro grenade for cheap.
I can't link it now as arnies is down for maintenance, but basically how it operated was the grenade was powered by a blank 12 gauge shotgun shell that would detinate after 3 seconds. This allowed the grenade to be easily recharged by popping in another blank shell and primeing it. It also produced an amazingly loud "Bang" that would be difficult to replicate with gas or spring powered grenades.
The advantage of this over a pure pyro design (such as with a fire cracker) is that you (the retailer) do not have to stock the explosives or even worry about selling them, as hopefully people can purchase the blanks from their local gun shop. Or heck, if you designed a grenade that was similar that used .22 blanks, you could even pickup the cartridges at home depot as they used them in construction.
The disadvantages are that you'd have to see about the legality of the device in canada (I checked with customs on importing, they stated that I needed to fill out a few forms and ask for a "ruleing" on the legality of it, never got around to it however), and their would have to be a 100% reliable way to ensure that a live cartridge would not be useable in the design. Also, i never did check on the availibility of shotgun shell blanks in canada, for all i know they may be prohibited and/or impossible to find. This could be changed by useing blank .22 shells as mentioned above though.
Also, with this design you most likely would not want to try and throw BB's into it due to the whole high velocity high temperature gas meets meltable plastic thing.....
However, if you designed a cheap (under 100 bucks per grenade and under say a buck a use) reliable and safe grenade that produced a noticeable "bang" (or alternately shot BB's in a wide area), i'd probably buy 2 or 3.
Edit: Found the link:
http://www.hfmgroup.com/professional...-grenades.html
It also appears to include an adapter for 9mm blank shells, which would most likely be easier to find than 12g ones.
You can see a video of it in action at this website:
http://www.combatcentral.co.uk/Image.../bangFinal.wmv .
It seems pretty damned cool