Pistol grip cavity cap for AR rifles: look around for the old rubber M1 carbine magazine caps at gun shows, etc. Heat them up with a hair dryer and stretch them over the bottom of the pistol grip, small tab to the front. They will conform to the grip as they cool down, retain their shape, stay put,and are well nigh waterproof. Unlike the aftermarket units, all the interior space is usable and they even improve the grip a little.
+1 for the weedwhacker line pull-through. Does most of what you need an Otis or bore snake for for practically nothing.
Also--Brownell's has a pistol cleaning setup by Accu Pro that has rod sections and a loop tip all inside the handle. There is extra room in there for jags, brushes, and other odds and ends. Very compact, very well made, and again, everything you need in a small durable package.
Scope covers: cut a truck tire innertube section across the round section, about 2" or so in width; just like a big rubber band. Excellent protection for the scope and the price is right.
Chore Boy copper cleaning pads: for serious leading problems use a small section (perhaps 1" x 2") and wrap around a worn bore brush. You can feel it
cut the lead away, and you can reverse it inside the bore to really get stuff out. Works as well or better than the old Lewis Lead remover with less effort.
Not something you buy, but if you are a revolver shooter, spend some time "skip loading" the cylinder for "ball and dummy" work. Load two chambers, skip one, load the third round. Spin and close the cylinder and then do all six hammer falls. The live rounds will come up in an absolutely random fashion to give you the best possible self-check on how good your trigger control and follow-through are. In addition to being excellent training for both beginners and old shooters, it also gives you twice the practice for a given amount of ammo.