<b>Oasis/Menagerie</b>
This one is pretty self-explanatory. Menagerie has issues both in the beginning of a match, when there are so many Copper, and also later once the deck starts greening. Oasis is a great cure for both. Plus, it adds some economy to an engine that doesn't want more than one of any single piece of treasure. The main issue with this combo is the opportunity lost with buying too many $3 cards, and also that it doesn't fully get around the need to trash. But, as with most engines, a +buy is usually required in order to justify putting together an intricate engine, and once the single Gold is bought that $6 is not as all-important and can be used to purchase two engine parts.
<a href=http://councilroom.com/game?game_id=game-20111203-224344-71d3c6a9.html>Here is a sample game</a>. I won the Ambassador and Familiar races with the help of this combo, and I was able to continually get good draws long after multiple Provinces were added to my deck.
<b>Shanty Town/Spice Merchant</b>
This is not a combo that I can point to and say, "Buy these two cards and ride them to victory!" Rather, it is a combo that you open with and use to cycle and trash early and often, setting up a strong mid and end game. Other components are clearly required, especially treasure. If the deck that this combo appears in has no terminal action, the Shanty Town becomes a virtual Laboratory, which is a great $3 opener. The "Lab" effect of Spice Merchant is relatively weak when it is used, but it improves all later hands <i>so long as ample treasure is added in a timely manner.</i> Combined, two seemingly weak effects create a surprisingly strong environment to improve, distill and cycle a deck.
After I had <a href=http://councilroom.com/game?game_id=game-20111202-213043-757373e9.html>discovered this combo</a>, I went into solitaire mode to sample it, and I have found that it is pretty good with big money at getting to four Provinces by turn 14, and with some Kingdom support it can improve even more.
<b>Hunting Party/Saboteur</b>
In the last several months I have seen this <a href=http://councilroom.com/game?game_id=game-20110911-164546-3211b316.html>devastating combo</a> in action a few times, and have been on both sides of it, and I have yet to see it fail. DG <a href=http://councilroom.com/game?game_id=game-20111110-091601-625cc79e.html>used it against me</a> in tournament play to win our set, and I <a href=http://councilroom.com/game?game_id=game-20111201-115157-cfbbccac.html>used it a few days ago</a> with the same result. I see people misplay or ignore HP often, so as a tip for you youngsters I'll spell it out: Keep it simple. Keep the number of unique cards in the deck as low as possible, avoid at all cost cards that draw, buy only one strong terminal, and muster as many HPs as you can as quickly you can. This takes some luck as well as skill, as bad draws in the first five or seven turns can doom this engine. One Gold is usually enough, and extra Silver buys early usually do not harm this engine too much.
Saboteur is great if you can play it enough to really put a dent in the opponent's purchases, otherwise it is pathetic. HP makes it happen a lot.