Not so much a hobby just something I picked up recently and have been playing with....
Bought a cheap stick welder at a Car Boot (still in its box never used). Got a bucket of scrap from a nearby blacksmith and have had a lot of fun trying to stick stuff together A Team style.
You get through the rods rather quickly but they're not that pricey if you stick to the common types. If you've access to scrap it's a pretty cheap and fun way to spend an afternoon.