Well, currently there are multiple things that can make you gold fairly easily. Here are some of the ones that have been lucrative for me. Word of warning though: It is worth noting that the expansion is drawing to a close and most likely the demand for the mats I farm will drop, dragging the prices down with it.

1. Cataclysm Raid Runs
The gold the bosses drops is based on the difficulty and raid size setting and the number of characters in the raid instance. On 25HC this is around 125g per toon + loot items (I would say the average is about 80g of vendor value). So, for example a 5-box would earn about 700g per boss. With a total of 27 bosses available this would mean about 19k from the bosses + whatever you pick up from trash mobs. Other raid tiers do not drop nearly as much gold, and I would assume this will get nerfed eventually as well.

2. Barns
A building in your garrison that allows you to go trap beasts that you then use for workorders that give leather/fur and savage bloods. As you can trap for a party of 5 at the same time (1 trapped beast = 5 toons get "tokens"), this scales nicely for multiboxers. Also the active time to keep the work orders rolling is fairly low. I do these on 2x teams of 10, and I still average about 40k every 4 days.

3. Felblight
Personally I'm not aware of any very good way of farming it, but killing Supreme Lord Kazzak weekly will net you 10 felblight per character on average. Depending the realm these go for a decent price. On my realm they go for about 80g each (2x10x10x80=16000g per week).

4. Tanaan Rep Tokens
The 4 rares in Tanaan Jungle (Deathtalon, Doomroller, Terrorfist and Vengeance) have about a 10% chance to drop a rep token called Medallion of The Legion. On my realm these go for around 3-4k each (used to go for like 15k... good times). I've spent a few lazy Sundays camping the spawns and I can usually catch all 4 in about 2 hours while watching Netflix.

5. Pet Farming
There are absolutely loads of battle pets in the game that go for absurd amounts of gold. Some are harder to obtain and some are silly easy for their price.

6. Boosting
Yes, yes, we know... No one likes the boxer booster. But if you can get someone to pay you 100k gold for a boost in Colosseum. It might be hard to pass up. *Wink* *Wink*