As a child who could neither afford expensive fishing equipment nor even look at it without it getting all tangled up and falling to pieces, crabbing was (and still is on occasion) a favourite pastime for me that did not require expensive equipment or (I hope) a fishing licence in the UK. Crabbing is exactly what the name would suggest, the catching of wild crabs.
What you need
Crabbing isn't as elaborate equipment-wise as fishing, but there are a few things you need:
Rope is the crabbing equivalent of a fishing rod and line, rolled into one. Any sturdy rope will do the job but bear in mind that the rope can get quite slippery, leading to a chance it might slip out of your hands and into the water, so it might be an idea to use a rope made out of a natural, bio-degradable material like hemp than one that isn't such as nylon.
A rock or brick
River/creek banks are usually quite well-supplied with decently-sized rocks but personally I would always use a fragment of house brick, as the holes in the middle of it allow for it to be more easily tied to the rope. Failing availability of a brick though, any rock that will weigh the rope down to the bottom of the water will suffice.
Crabs are carnivorous/scavengers and so a good choice of bait usually is some form of dead meat. Personally I used to use frankfurters, but I've seen other crabbers use fish, and quite perversely, smashed up pieces of other crabs. Crabs of course have rather powerful pincers so make sure your bait is quite thick.
A bucket of water
Or more accurately, a bucket of water that you've filled from wherever you are crabbing. Crabs can feel quite annoyed when caught, so a good idea is to put them straight into this bucket rather than holding them for photographs or whatever. Throw them back into the water as soon as possible though. (eating them is most likely illegal if you're on a local river or creek)
Again, setting up is very easy when put next to fishing. All you have to do is tie the rope around your weight and bait, find a nice spot on the edge of a body of water known for its crabs on a nice sunny day and hey Presto! You're all ready to go.
Getting down to business
Well, now you are all set up, it is as simple as throwing your rope in the water and periodically pulling it in to see if you've caught anything. There are many styles of rope-throwing, most notably "The Cowboy", (swinging the rope over your head like a lasso before throwing) underarm and overarm. Be careful though as careless and over-energetic slinging of the rope when it has been thrown a few times and is starting to get slippery can cause it to go flying into the water, causing you to fart on your Weetabix and lose your equipment. Many crabbers like to spend the time waiting to pull their rope back in with other pursuits, namely drinking lager from their chill-boxes.
When you have caught a crab, put it into the bucket of water and give it a while to let go of the bait, before pouring it back into the water.
When you're all crabbed out, just make sure you've released all of your catches and don't leave any litter. No fuss and no untying various knots as in fishing.
That's all there is to it really, and is how a lazy, unskilled fisherman such as myself gets a good excuse to drink beer on the edge of a creek on a summer's day