I use the Park Tool cog brush. It's the only special cleaning tool that's worth the money, IMO. Toothbrush bristles are too short for getting in between the larger cogs. The Park Tool brush has long bristles and a narrow profile, and slips right in. Toothbrushes are nice for the rims and spokes (although one advantage to frequent cleaning is that the spokes usually clean up with a rag).

For the narrower spaces between the pieces of the brake calipers I still find that a shop rag or folded heavy paper towel works better, though. And there are some of those nooks and crannies in the derailleurs, particularly, that I just use a narrow punch or a pick to manipulate a heavy paper towel (the "Shop Rags in a Box" type, which also come in rolls at the hardware store).