1. Heroes' Guild - 20 keys - The Murren Greathammer
You'll see this one almost at the very beginning of the game. You won't be opening it anytime soon, though.
2. Greatwood Lake - 5 keys - Elixir of Life
It's right in the middle of the area. When you get there, you should already have the required number of keys.
3. Cave Larder (inside Hobbe Cave) - 5 keys - Will Master's Elixir
Enter the second door (on the left) from the Cave Entrance.
4. Darkwood Lake - 15 keys - Arken's Crossbow
The Crossbow we find here is quite nice indeed, but it doesn't stand a chance when compared to Skorm's Bow.
5. Grey House - 10 keys - Sharpening Augmentation
Go to the abandoned cottage and upstairs.
6. Witchwood Cave - 15 keys - Health Augumentation
It's in the cave where you found the missing Archaeologist.
7. Headman's Cave - 15 keys - Mana Augumentation
You'll get to this one only while performing the Mayor's Invitation quest. It's in the cave, when you cross blades with Thunder.
8. Bowerstone Manor- 15 keys - Katana Hiryu
You'll have to either marry Lady Gray or expose her evil deeds and become a mayor to get in here (bronze quests' section).
9. Circle of the Dead - 10 keys - Piercing Augumentation
This one is easy to find, you'll surely see it while following the Graveyard Path quest.
10. Hook Coast - 15 keys - The Murren Greataxe
It's at the top of the lighthouse.
11. The Necropolis - 25 keys - Archons armor
It's near the river, on it's western side.
12. The Lost Bay - 30 keys - The Avenger
You would really expect a 30-key chest to contain something cool. The legendary sword Avenger, however, isn't anything special at all.