Short list of things that can be traded for emeralds with villagers:
- Wool (any color, but any one trade must be a stack of a single color)
- Raw Beef
- Raw Pork
- Raw Chicken
- Cooked Fish*
- Wheat
- Paper
- Books
- Coal or Charcoal
- Gold
Theoretically that list could expand to diamonds (Ha!), iron, a written book
*This one is especially important, since it's the last trade for a certain villager
Short list of cool items that can currently be bought with emeralds from villagers:
- Glowstone
- Bottles o' enchantment (experience in a can)
- Chain mail helmets (not really useful, but unusual since they can't be crafted)
- Eyes of Ender (can be used to make Ender Chests)
Crap trades that aren't worth the emeralds:
- Diamond Hoe (seriously, who needs a diamond hoe?)
- Compass
- Clock
- Bookcases
- Assorted lame enchantments
- Food
- Leather goods
The exception to all of these is if they're the last trade on a list. Then they might just be worth the cost. Spending 1-3 emeralds to allow trading for a half-dozen or more is worth it.
For those that don't know how trading works, here's a short rundown of what I know:
1. Each villager starts with one trade.
2. Making that trade a certain number of times (sometimes just once) will upgrade them to have another trade.
3. The total number of trades a villager will have varies.
4. Each trade can only be made a certain number of times, but making the last trade on a villager's list will reset previous trades.
5. So, if a villager has a desirable trade X-ed out, or refuses to trade anymore in the middle of a session, then make the last trade on his list and he should be willing to make all of his trades again.
Cautions!
1. Villagers love to run about and knock each other around. If you're trading with a villager and they get knocked far enough away that your trade stops, you might end up punching them. Punch too many villagers and iron golems will slaughter you. When trading always click slowly and deliberately. (Also, be careful throwing eggs.)
2. Since trades can run dry until you make the last trade on a list, sometimes you have to spend emeralds to be able to trade for more emeralds. Try not to run out of emeralds so it's always easy to open up new trades.