Basically, the idea is that you can take a slimeball, and put it on one side of a block. Then, if you put another block next to it, they would be stuck together so a sticky piston could push and pull both of them. It's simple, but I think it would be very useful.
you cant rly throw down a block and have it stick to something right next to it, so you can have a piston press it to the glue though. i have thought about this idea b4 but im glad someone has brought it up :3
...Only on Tuesday
or
http://www.minecraftforum.net/topic/438214-sticky-block/
Mostly moved on. May check back a few times a year.