Well, being at "end game" in Diablo, I have been doing a decent amount of farming. First, I have been farming The last part of act one on Inferno with random groups. I die a lot....however, I get a metric crap ton of loot and gear to sell.
If you are going to do act one of Inferno, start with the quest where you are going to go rescue Tyrael and have to go through the Halls of Agony.
Make sure that you kill enough rare or elite mobs to get five stacks of Nephalem Valor and then proceed to the Warden. Kill the Warden and them move on the Butcher. Simple. During this run, you will get a ton of blues and yellows. I had to sell my blue items off at least half a dozen times and I ended up with around ten yellow items for the run.
When I am soloing, I just solo in Hell difficulty. It is incredibly easy once you have some decent level 60 gear. I have been running through act one and making a couple hundred thousand gold in the time it takes me to get to the Skeleton King and Kill him. Nephalem Valor applies to every difficulty once you hit level 60, so farming in Hell will net you a lot of gold and items to sell. Today, I will solo the second half of Hell to see how the gold per hour is on that.
Now onto the subject of gold. What is it good for? Well, the only thing it is really good for at this point is buying the best gear you can. I just bought a pair of pants today which were an upgrade in strength, vitality, and gave me physical resist and resist all of 200k, which was about 25 percent of my gold. It is not like I can buy things such as pets and mounts in Diablo. Gear is the name of the game.
Speaking of gear, Resist All is the stat of Kings apparently. The more you have, the easier the game will get for you. Unfotunately this gear is expensive, so go out and far the crap out of that gold.