I think another thing worth mentioning is that when a card has some of its benefit on buy/gain (or just on buy in farmland's case) then once its in your deck its less valuable to than a card of its price ought to be. This makes it a good target for trash-for-benefit like Apprentice or Bishop.
By this category of cards I mean things like Farmland, Border Village and Ill-Gotten Gains, all of which have strong effects as you acquire them, but then later on become a dead 2VP card, a village and a weak silver respectively. Hitting these cards with a trash-for-benefit card gives you lots of benefit for not much deck value lost. In comparison, if you Bishop a Laboratory or a Gold, then you weaken your deck a lot more.
This isn't so much a combo, more just a general thought to bear in mind when formulating overall strategy for a kingdom. You can Farmland that Estate into a Bishop, and later Bishop that Farmland. You can Farmland that Silver into an Apprentice, then later, Apprentice that Farmland. Or to put it another way, when you buy Farmland you get $8 worth of Benefit for a $6 price tag, so long as you have the means to reclaim the $6 that is tied up in the card.
Overall though, I think the main use of Farmland is for brinksmanship purposes: setting things up so that you have the edge when scores would otherwise be even or marginally in your opponents favor. The classic use is to buy one on the last turn on a three gold hand, Farming a Gold into a Province, and thus getting 2 more VP than you might otherwise have had. Works for Colony games too, if you're looking at Platinum + $6. Once or twice I've even found that Farmland is a good way to three-pile a game, when emptying a $4 pile will end the game, but you also need the 2 VP to get ahead as the game ends. Its a situation that has come up more often than I might have expected, and generally worth being aware of. Mostly then Farmland is a bit like Chapel, in that its card you probably won't want multiples of in your deck, but whose presence you have to be aware of when you're watching the game's tempo.
I'd agree also with using Farmland to get rid of action cards that no longer serve a purpose for you. I find Moneylender and Spice Merchant often diminish in usefulness in mid game, and with a hand of $6 plus one of those cards, its better to Farmland them into gold rather than just buy the gold.