Kakuro at Cool Math Games:
I thought this was impossibly hard at first.
The key is to fill in all single squares first, then look for doubles and up.
Cool that you can ask for hints!

INSTRUCTIONS
Fill in all the squares on the board following these rules:
1) The numbers in vertical and horizontal row groups (not entire rows) cannot repeat.
2) The numbers in each horizontal group must add up to the number that appears in the triangle at the far left of the group.
3) The numbers in each vertical group must add up to the number that appears in the triangle at the top of the group.

HINT: Fill in all the single group squares first! See if these lead to any double square groups you can get and so on.