I'm probably going to close this thread as there is a NUMBER of resources on grip work already available... but first I'll do my best to contribute. Since you stated NO goals whatsoever I'll cover what I think are the best, most broad methods for developing forearm strength. In no particular order:
Thickbar work. If I had my way every gym would be equiped with a range of barbells and dumbells varying from 1" to 3" thick in 1/4" intervals. Everyone would then evaluate each exercise thusly, "Can I hold onto the bar easily while I perform this movement?" if the answer is yes, you need a thicker bar. This goes for deadlifts, bent over rows, pullups, and especially isolation exercises. I also contend that all olympic lifts for non oly lifters should be done with a thickbar.
Rope climbing. Get a thick rope (mine is 1.5" manila, but if you can find 2" or greater) and string it up on a tree branch or something. now climb that mother fucker! Good for finger strength, wrist strength, thickbar sustaining strength and it hits your upper body to boot.
Thick handled KB's. I made my own (pictured below) for about 50 bucks (not includning plates). Now do swing and catches with it: perform a swing but change hands by tossing it in the air and catching it with the other hand. Carefull, I strained my back with this one so work on form before you go super heavy.
Block weights. Brookfield (owner of some of the strongest mits the world has ever seen) says blockweights = hand strength. His word is good enough for me. you can do indivual finger lifts with them, blockweight tosses from hand to hand (keeping an overhand grip), snatches, cleans and presses, etc. Blockweights are a great tool for hand strength. Don't know what a block weight is? check the glossary.
Levering = wrist strength. Grab a sledge and lever that son of a bitch. I would argue that this kind of wrist strength (front levering and overhead levering, as well as rotational) is the most important lower arm strength to develop for fighters. If you can get a solid grip on your opponent you help move your opponent with the strength you'll develop with levering. and it will make your punches much more solid.
You'll note I didn't mention grippers on here? that's cause grippers are good for getting better at grippers. that's about it. If you want to get better strength for cutting nails or wire or clamping down pliers or something, fine, grippers are good. But if you want good grip training for any other athletic endevor, you may want to consider hiding your grippers and focusing on other things.
Lastly, learn to bend shit. It's great for parties and bending nails is the greatest expression of lower arm strength in existence.
