I still need to play with these a lot more to commit, but here are a few (but not all!) of my favorites so far. While the powerful, expensive cards are great, I'm enjoying the subtle utility of the cheaper stuff.
Port - I did not expect this to be as good as it is, but Port is fantastic. Getting all of the Actions you need is just so fast with Port. A lot of the penalty of playing an engine versus Big Money is in how much time you have to devote to picking up pieces, particularly before you get +Buy. Not only do you get Ports twice as fast, but it enables you to start stringing other pieces together in order to get to your other buys faster. So simple, but so effective.
Coin of the Realm - It's not that Coin of the Realm is extremely strong, but that with minimal investment in one or two of these, you have a huge safety net for a deck drawing engine. That alone is cool but I think where it really shines is big money. Why? Two reasons - the impact of a Copper is lower, and it lets you add several more terminal actions to a BM deck and string them together on an as needed basis. Plus you only commit to using it after you draw your cards if you're playing terminal draw!
Gear - The potential of Gear is absolutely enormous. It's almost Scheme like in how it prepares you for your next turn. It's everything Haven ever wanted to be and more - and at worst, it's a +2 Cards that lets you set aside terminals for next turn. Gear is so, so, so good.