As for myself, I have two low cost gadgets that are in my bag:
The first is just a bit over the $25 price cap, but I think it bears mentioning here because it's incredibly useful and is also along the lines of Eduard's suggestion. I use Professor Kobre's
LightScoop, which weighs in @ $29.95
This handy do-dad attaches to your camera's hot shoe and has a built-in mirror that deflects the flash beam and bounces it off the ceiling or wall and back towards your subject for softer, more diffuse light. Say goodbye to red eye and harsh shadows from overpowering flash units!
My second gadget is the Nikon
ML-L3 infrared remote, which costs around $15. This thing is great for reducing camera shake and avoiding the awkwardness of using the camera's timer function.