Sugru starts out slightly sticky, like Silly Putty, but after a few hours solidifies into a harder rubbery texture which retains its shape but is still pliable. It can be used to fix any variety of things, or to "hack things better" as the Sugru motto suggests.
This instructible details how to fix the silicone rings on the earbud headphones that come with the Apple iPhone.
Step 1: Start with broken earbuds
The rubber rings on both sides was separating from the plastic. That's bad, because obviously it looks bad, but also because the lack of ring leaves a slight sharp edge on the surface which was beginning to irritate the inside of my ears.
Additionally, one side of the earbuds was coming apart, the two plastic parts no longer glued together.
You can see what remains of the rubber from one side in the photo.
Step 2: Get some Sugru
Each packet contains one of four colors, black, orange, green, or blue.
A single packet has far more Sugru than needed for this repair, so I did my best to seal up the rest of it to use for something else.