The deep-groove cheap rubber floormats are a great mod. On the same aisle at the *Mart of your choice, we bought two visor covers: the left one has CDs, a place for a pen, and a place for sunglasses, the right one has a zip pouch (holds a few documents well, like the registration), a slot for a pen, and a mesh pouch that holds random little thingies like chapstick. Total cost was less than $20 for the two.
If you already have a Tuffy console, a cheap mod is to get some rubber non-skid at the hardware store and line the bottom of the tuffy and the cupholders with it. Makes stuff quieter, easy to clean.
Extend diff breathers. Takes little time, cost is just a few yards of fuel line.
On an ARB bumper: weld on a hilift point to the corners. We used some scrap 1/2" sheet, cut a hole out for the hilift and just welded it on. Cost about $4 but that was at a little shop in San Felipe, Mexico.
Fire extinguiser mounts nicely below the driver's seat.
Get a little magnetic base bowl (I think we got ours at Summit as a freebie with an order). I stick mine to the back of the wheelwell, inside the truck. It always holds onto at least a few screws or bolts as they rattle out or I don't quite finish re-installing something...
Better speakers up front. We bought some at Circuit City or somewhere for about $40/pr. Huge improvement in the sound quality. 20 minute install job.
Small bungee cords (the really small ones) under the seats help keep the toolkit and other junk from falling out when you open the door. Less than $2
Paper towels: take a bungee cord and stretch it from the driver's side grab handle to the handle in the backseat. Slide a roll of blue shop towels over that. Towels are always in reach, stay dry(er) and clean(er) than anywhere else I've tried to stuff them. Many thanks to Tom Boyd for this free mod (assumes you own a bungee cord and shop towels)
Hood lock: a length of chain or steel cable can be strung through the holes in the front of the hood and draped behind the grill (in front of the radiator) to the frame rail, where you can lock with a padlock. $10.
RainX antifog. Applied to the inside of the windshield. Helps.