Wow, this is a really good informative thread. It does seem that a guy can't have enough tools. I've learned with many Cubs that a battery charger (already listed) comes in REAL handy. Punches, BFH's, pliers, screwdrivers, all sorts of sockets and specialty wrenches, welders, a torch (which I need), three jaw pullers, test lights, calipers, mulitimeters, and impacts, all just to name a few that have already been named.
For hardware, I keep everything that comes off a tractor. Screws, clips, all lengths and sizes of bolts, round bar stock, scrap metal, etc. Im a hoarder
Like Mike said, try buying a few parts tractors and resist the temptation to get them running. By far a really good resource for parts. I used to get every tractor I bought running, not very stable financially. Finally parted out rough tractors and now I have two small sheds worth of parts. Belts, carbs, SG's, clutches, gauges, electrical, engine parts, PTO's, gas tanks. Some stuff you probably (and hopefully) would never need. But, in that one instance, it might just come in handy!
Good luck, and keep the great ideas coming!