I have two hobbies that kind of pay for themselves, or at least contribute a bit of cash flow. Both are biological in nature - aquariums, and gardening. What's kind of cool about both of these hobbies is that you can literally create something from nothing *it's ALIIIVE, mwuahaha*. No, literally, you just pop a couple of seeds in the ground or buy a couple guppies, and all of a sudden without thinking you have something of value.
Given that I am an avid aquarium hobbyist. I utilize my aquariums as both my visual pieces (I have aquariums in place of my TV, artwork, and as focal pieces of my living spaces) as well as light sources in the rooms they are in (if I have to have the light on to see, might as well be having it serve several functions instead of just for visualization).
I have both saltwater and freshwater tanks. The freshwater tanks have provided a bit of income for years. I have bred a lot of tropical fish, and have also grown a lot of aquatic plants that I can then either resell on Craigslist/Ebay (check out Aquabid for some of the more outrageous sales regarding rare fish and plants) or trade in with my local fish store for either a discount, profit, or a favor.
I've sold some of my excess guppies, swordtails, and cichlids for a couple bucks here and there. Common aquarium fish aren't all that practical, but one can raise some of the more prolific fish for generations on mere pennies for food, heating, and water changes (instead of having to buy new fish every generation). Some of the more pricey fish, such as the African cichlids from Lake Malawi and Lake Tanganyika have a decent market, and one would be able to sell juveniles or adults for a bit more if you can find a market for them in the local area.
Aquarium plants are also an interesting venture, since some of the more lucrative items can be shipped around the country or even overseas. Creating a business would probably require a semi-tropical environment and acres of greenhouse space in order to turn any sort of profit, but I can make a few bucks here and there from extra trimmings or rootings of plants, as well as trading for new plants, fish, or supplies.
I just started a small 2.5 gallon saltwater reef tank with an LED light bulb. My idea was not only to just have a reef aquarium for my personal fascination, but it again, serves as a desk lamp for ambient lighting, as well as a propagation tank that I will be able to sell coral fragments or "frags" from. A small 1x1" frag of coral can bring in $20, and for some of the more quick growing corals this can be a very easy way to make a little cash, since with the new LED technology electricity use for lighting is very low (compared to metal halides or fluorescent bulbs) and the saltwater mix is mere pennies for a small saltwater reef.
The freshwater tanks, in addition to providing me with pleasure from watching my fish breed, grow, and flourish in an artificial environment, also provide me with nutrient-rich water for my houseplants and my gardens, which is the second hobby of mine.
Gardening is awesome, since it allows me to spend time outside. I cut down on the time spent on my lawn watering, feeding, weeding, and mowing since I am slowly covering it with hardy perennials. Composting reduces the waste generated from the kitchen, aquariums, and junk mail. Veggie gardening in an efficient manner allows me to cut down on the costs for my food, enjoy fresher produce, and give me satisfaction at accomplishing such a primal and simple task.
It's almost alchemical in nature. I can pop a seed in the ground, water it and watch it, and it grows before my eyes. I might start referring to my garden as my "Green Mustache" this summer, because just like some investments, it automatically throws off great benefits with a little bit of work and thought.
But really, it is magical. I can take a plant, such as a catnip plant (Nepeta sp.) and divide it into several sections, then plant all those sections. Give it a season, and I'll have 4 plants of the same size. Give me a box of sand, some rooting hormone, and several stems of the catnip plant, and I'll have 100 little plants that will grow to the same size in two seasons. That sunflower I planted? It just generated 100 new little seeds that can then become a 100 little sunflowers that will generate 100 little seeds themselves the next year! I could sell the seeds, give them to a new gardener, or swap the sunflower seeds for something different online.
Given that perspective, every other kind of hobby seems kind of expensive in a sense :)