Age 3
Age 3 is host to a big number of screwing up/stealing cards, making for a pretty weird age. Good cards from age 3 include Machinery (which lets you steal all your opponent’s cards), Education (which lets you make big jump in tech), Optics (like Sailing but with added chance for scoring) and Paper, another card that lets you draw huge numbers of cards.
Scoring
Engineering - The problem with Engineering is simple. It’s a castle based attack against castles. So Engineering is better as your opponent gets more castles, but the more castle cards he has, the harder it is to successfully use Engineering. As such I personally don't like the card that much. It also lets you splay red cards left as an added bonus which you should consider.
Tech Up
Alchemy - Alchemy is the first "risk for a reward" tech up cards. Luckily that risk isn't that big, as out of the ten age 4 cards only two of them are red. The fact that it also draws and melds is pretty big too as it spares you an action and it also lets you score from your hand. So all in all you have a pretty neat package in a single card, although it risks blowing up in your face. Of course, you can use it offensively by intentionally sharing it with an opponent who has lots of cards hoping they’ll hit a red and have to return their entire hand. Further, if you have 2 or fewer castles, it safely lets you meld and score from hand in one action, which can be useful in a pinch.
Education - Education lets you return your highest card from your score pile, and then draw a card two higher than the remaining card in your score pile. It can be as powerful as Math. But Math does all his job alone, quickly and efficiently. On the other hand, Education needs some support, and so it can be pretty hard to use. If you are using Agriculture you can do jump as big as 3 ages with each cycle of Agriculture/Education. Optics puts a 4 in your score pile without you doing much work, which makes it another good card for Education. And if you use Democracy (an age 6 card) you can jump from 6 to 10 in a single swoop which is huge.
Board
Optics - Optics is like sailing but it draws and melds age 3 cards instead of age 1. If that cards comes with coins you score a 4, which is awesome. Iif that card doesn't come with a coin you give an opponent (who has less than you) a card from your score pile. This might sound bad but if you lose just 1 point it's not the end of the world. If you combine it with Mapmaking you can even score with 100% chance. All in all, Optics is an awesome card.
Drawing
Paper - If there is a reason to splay your colors left, it is this card. Just like Fermenting, drawing 5 cards per turn is huuuuuuuuuuuge. Unlike Fermenting, splaying all 5 colors is a lot easier than gathering 10 leaves on your board. Also unlike Fermenting, it lets you draw age 4 cards rather than age 2 cards. Even if you don’t manage to splay all your cards, it lets you tech up. Best of all, Invention from age 4 is great combo with Paper and makes for an incredible 1-2 punch.
Stealing/Board Screwing
Compass - Most of cards that screw with your opponent’s board give something to your opponent as compensation. Of all these kinds of cards, Compass has the best compensation. If ever card on your has leaves, that compensation can nothing at all. Sometimes you can leave a bad card, non-leaf card on your board and is it to cover a useful opponent top card. That said with the exception of Fermenting, there aren’t leaf cards in the early ages that are bothersome enough to be worth stealing with Compass. But as tech gets higher, the cards that Compass can steal become much better. It can steal cards like Canning, Lighting, or the myriad of yellow cards that tampers with the opponent’s score.
Feudalism - By age 3, finding an opponent with a castle card in hand is a quite hard task. But on the other hand, it can steal both Machinery and Alchemy, so it's not as bad as oars. The important part of the card is that it lets you splay both purple and yellow cards left, which helps you set up big draws with Paper.
Machinery - If there is one card that you should be scared off when doing massive draws, it's this one. Stealing all the cards in the opponent hand is huuuuuuuuuuge. Most of the time, giving one of your 3 for 5 or 6 1s is an awesome tradeoff. To top it all, it lets you score a card with a castle from your hand. It also lets you trade an empty hand for anything your opponent has, which is incredibly annoying to your opponents.
Medicine - Medicine lets you screw with your opponent score and eh, it's okay. For optimal use it requires a situation where you scored a lot of small cards and your opponent has scored a lot of big cards. That doesn't happen often. The best part of Medicine, after you give your opponent a sucky card, you can then get it back by Mapmaking/Navigation. Note that if you have no score at all, it will let you steal a card for free. This combos with cards that require you to return a card from your score pile to activate.
Special
Translation - Translation lets you meld card from your score pile. If you have any of the big cards in your score pile (like Math for example) it might be worthwhile to sack all your score for your board. But most of the time it's not worth it. Most of the stuff said for Canal Building applies for Translation too. The other cool thing about Translation, it nets you the World achievement if all your top cards have a coin. Occasionally, you can use the first effect if you to get the achievement. If you are in a situation where one of the cards in your score pile has a coin on it in a color that your board is missing, activating Translation can get you the coin coverage needed. This is quite situational and only easy to achieve when you have a small numbers of piles, or better yet, have only Translation on your board. But that puts you in a bad position against Clothing, so it's hard to get this off easily unless you pull it off with Tools.