Borax slime is really easy and inexpensive to make, and kids absolutely love it.
All you need is:
1 tsp borax
1/2 cup Elmers glue (white or clear both work)
food coloring (optional)
In your first bowl combine 1 cup water with 1 tsp borax, stir until dissolved. set aside.
In your second bowl combine 1/2 cup elmers glue and 1/2 cup water. You can add food coloring to it if you want. Stir until mixed well.
Now add your water/glue solution to the water/borax solution. Your slime will begin to polymerize immediately.
Stir for a minute or so, and then begin to knead by hand. Drain the excess water for the bowl. The slime with start off as a highly flexible polymer. You can stretch it and watch it flow. My kids had a good time having slime races, by putting it on the edge of the table and seeing which one touched the ground first. After a while it will become stiffer and more like putty.
The slime won't stick to most hard surfaces, but it will stick to clothing and carpeting...so be careful. :) We've found that it washes out of clothing well, as long as you don't use too much food coloring. The food coloring can stain surfaces.
The kids had fun placing it on this wire table and watching it ooze through. When you lift it up, it keeps the design for a few seconds.
You can keep your slime stored in the refrigerator in a ziplock bag.