Fishing

Fishing is one of those secondary WoW skills that many people just ignore. They think it's pretty boring. Well, actually it is a little but you can make 30 to 40 gold and hour at character level 20. Most of the fish you catch will be vendor junk but a few fish have tradeskill uses and sells well in the AH.

The first two won't make a lot of gold but are good for a little extra spending money while you skill up.

Oily Blackmouth sells for around 10 silver each on most servers. Best fishing spots for these are:

Menethil Harbor, Wetlands
Southshore, Hillsbrad Foothills
Zoram'gar Outpost Ashenvale
The Merchant Coast, The Barrens
Near Northwatch Hold, The Barrens
Baradin Bay

Firefin sell for 5 silver each and can be caught in many zones. My favorites are:

Blackwolf River in Stonetalon Mountains
Dreadmurk Shore in Dustwallow Marsh
Western Strand Fishing in Hillsbrad Foothills

One your fishing skill is well into the 200's, you can start raking in the gold. Fish for Stonescale Eel, the price depends on the server as always but 1 to 1.5 gold each is common. You can easily get 20 to 30 an hour fishing in these spots:

Bay of Storms in Azshara
Feathermoon Stronghold in Feralas

A special mention for Deviate Fish which sells for about 20 silver each in Horde AH and up to 1 gold in the Alliance AH on PvP servers.

The fish can be caught in any Oasis in The Barrens which is why fishing there isn't a popular pastime for Alliance characters on PvP servers. One little know fact however is that the Wailing Caverns instance is on the same fishing loot table. Once your Alliance character is 30+, you can happily zone into WC kill the 3 or 4 mobs in the entrance and fish all day undisturbed.