I found an old jar of Spackle that was hard and clumpy but rather than throwing it away you can make it as good as new in 30 seconds.
Step 1: Water
Add a liberal amount of tap water. The jar I had was about half full of dried spackle, I filled it to about 2/3 full with water.
Step 2: Mix
Break up the dried clumps and mix it with the water just a bit. You won't reconstitute it but you just want to get it so the water isn't just pooled up on top.
Step 3: Nuke
Put the jar in the microwave for 20 seconds. After 20 seconds take it out and mix it a little bit more.
Step 4: Nuke It Again
Put it back in the microwave for another 30 seconds. Afterwards take it out and mix it thoroughly. You should have a nice mixture of ready to use spackle.
If it's still too watery drain out some of the water and microwave it for one more round.