There is an incredible list of materials out there which one can farm to generate income. While I don’t usually advise farming because I think there’s faster ways to make money, one thing that will always do well for you is Spider’s Silk. Prior to Cataclysm, it was an item with a moderate value for its level and availability. Since Cataclysm, the availability has decreased, while the demand for it has not.
As well, there’s hardly anyone farming it. I would hazard most people get it accidentally. There are three zones where mobs will drop the silk though I’ve found Wetlands to yield the best results (luck may vary). Just West/Northwest of Slabchisel’s Survey, or immediately left when you’re come out of the tunnel, is where you’ll find Leech Stalkers. Spider’s Silk seems to drop from them at a one in four or five rate.
So the next question is what to do with it, right? Well, you could just put it up on the Auction House as is. On my servers I see it bouncing regularly between 10g and 30g a piece. Not a bad return at all for a little bit of work in a lower level. However, if you have a tailor (or access to one) you can really profit. It takes 4 Spider’s Silk plus some Bolts of Silk and Medium Leather to make Spidersilk Boots. These are the “best in slot” boots for caster twinks at level 19 (and non-twinks will get another 10 levels out of them easily). I put up pairs of these one at a time for 120g-170g each and only about 15% of the time does a pair not sell. That’s clearly just a case of timing, because the other occasions they are gone like lightning. Doesn’t matter if it’s Horde or Alliance either, and the fact that it’s a recipe tailors receive from their trainers seems to be irrelevant as most don’t bother making them.
Like other suggested materials, you’ll also want to keep an eye on Spider’s Silk on the Auction House as quite often it will pop up with people not knowing how profitable it can be. Quite often I can get it for one gold or less, but somewhere around 5g a piece is where I tend to purchase the most. Your situation will be different I’m sure, but in no time you’ll determine where the margins are on this little gem and if you’re a tailor you’ll be laughing as you count the income.