Below is a small sander I picked up off of ebay a couple years ago for $25.00. I use it a lot. Sticky back disc. I put a #80 paper on it for grinding down HO scale loco frames for DCC sound. Usually #150 paper for styrene or wood.
I sometimes use a Dremel along with the sander.
Go to ebay and search for hobbyist 4" mini disc sander
Rich
The Allwyn sold by Micro-Mark is a nice little unit too. It has both a disk and a belt on it, so you can do inside corners.
Pretty good bargain, too at approx $90 (including S&H).
I have a small belt sander I got at Harbor Freight. The idea was to use it with Fast Tracks turnout jigs, but I sort of gave up on that. TO date I have used it for one thing - putting an edge on a large putty knife to use to cute extruded foam. Works very well (the sharpened putty knife) - I even used to in the Home Depot parkign lot to cut the 8 foot long sections in half to fit in my car. Clean cut with minimal mess - and cutting foam can be VERY messy.
–Randy