What would you do with an ex drop? If you're like most people, you'll just spend it on something you want, and that'd be the end of that. Or maybe you'll lament how the only people who earn currency are the no life market flippers, and then spend it on something you want. But you know, there's more to it than that.
I'm going to sort this by methods, and of course you can do each at different levels of intensity. Pretty well everyone I know who's got significant ingame currency uses at least one of these. With a side of larceny on one memorable occasion*, but that was in addition to this, not instead of it.
1. Playing the game
At its most basic level, this involves selling magic/rare items for chaos and alterations, or even transmutes. Slow but steady. Alternatively, getting a couple of Perandus rings and selling id scrolls to people. One level up, and it involves selling drops for currency. The majority of saleable drops are from maps in the 72+ range, and most of the better drops in that set are from i75+ items. Which is just as well, because at that level, it takes a lot of currency to feed a mapping habit. One other method, though, involves running all maps from 72-75 and selling anything that drops both above and below that range. Between that and good drops, it's possible to break even on solo mapping and then some. If running 76+ maps consistently, the sale of ilvl whites for crafting is a good side income.
But wait, what about MFers, you might ask. And sure there are some people who farm Piety/Dominus with MF and play with the chaos recipe. But a dedicated MF support character isn't in the business of alterations - a typical map run with one of those involves a stack or two of id scrolls per person, rares dropped on the ground as you run once it's established that they aren't good rares, and multiple uniques per map. The good ones get sold, the bad ones get dropped, and yes, people leave uniques behind.
2. Playing the market
Buy low, sell high. The best way to do this is on currency and uniques, because the demand for those is guaranteed. You can do this the regular way, which is to know the market well enough to establish a range of prices - and do note that prices change by time zone: ex are 26-27 chaos at US prime time on Nemesis, but 28-29 at EU prime time. Then buy low and sell high. You could start small, with chaos/alt/fusing flipping, and move on to bigger game with Andvarius flipping, and at the end of this line are mirror and shav flipping.
Or you could find and exploit a hole in the market. In early 2013, 20q FP gems sold for 2 ex during the FP craze, but below that (in the 13-18 range) for several gcps. People made exalts per day upgrading and reselling gems. During the entire time Onslaught was active, death rush rings were in the range of 2-4 chaos on OS...and 1-2 ex on SC. I made exalts per day on that and the cross-trading forums (1 OS = 4 SC). I will not post any current exploits.
This is where I'm going to classify "running Dominus for people" - there's a market, you fill it.
3. Playing the slots
No kidding, crafting is a really big earner, once several conditions are fulfilled. One, you have sufficient currency to start and to ride out streaks of bad luck. Two, it's done on large enough scale that item sales take place relatively often. Three, effort has to be made to actually sell items. Once these conditions are met, however, one realises that others will pay them to bet against the RNG. The chance of spending 2-3 times someone's asking price on crafting a usable item for yourself is too high if you're going to just craft the one item, so it makes sense for you to just buy said item. The guy crafting said item, on the other hand, is rolling enough items that his luck averages out. For every bad luck streak, he's getting a good one - and his asking price is rather more than the average amount he spends. And ta-da, instant profit, both sides satisfied. (Yes, good luck streaks exist. I once spent 6 chaos rolling two pairs of ES gloves for about 3ex value each. I've also spent 4ex of chaos on a helm and gotten absolutely nothing worth selling. Go figure.)
This has gotten long enough, so I'll analyse the effects of this...and why I don't much favour SFL...in another post another day. But you can probably already tell from here.
*Don't ever trust Hauntworld with anything you can't afford to lose. He's a pretty cool guy to talk to, and really good at PvP, but don't trust him with your stuff. I've also heard some things about his guild, and from observation it seems to be more than plausible, but have no personal experience there.