The concept behind the CarrySpeed MagFilter is a fantastic one. For $25, you get a two-piece kit — a metal ring that is stuck to the front of your high-end compact camera, and then a filter which magnetically latches to the ring. That's it, simple, fast, and it'll allow you to use filters on your Sony RX100 or Canon S100, and easily pull them off when you don't need them. These cameras have enough of a following with expert users that you can bet some people are begging for filters to use when out shooting.
Right now, CarrySpeed only has a polarizing filter, but word is that a neutral density is in the works, too. There's a 42mm version that'll work with the Sony HX9V, HX10V, HX20V, HX30V, and RX100, and a 36mm edition for the likes of the Canon S95 and S100 — but with a free downloadable template, you can see if they'll work with any other gear.
The downside to this project is that you're stuck with CarrySpeed's filters, since it's a proprietary mount. You can't go out and grab a filter that you already know and trust, so you'll have to hope that the glass that CarrySpeed sourced is of good quality. Alternatively, there are products like the Lensmate Adapter, which gives you a standard, screw-mount adapter for filters, which opens up a wider world of existing products.