You can fix fraying wires, hang pictures (without a nail), create ergonomic grips on tools, and soooooo much more.
It sets strong because it turns into silicone rubber, permanently bonding to almost any surface. It's waterproof (use it to secure a shower caddy!), heat- and cold-resistant (use it to insulate a pot handle!), and electricity-insulating (fix those constantly fraying smartphone cords!).
Promising review: "My Macbook Pro charger wire was starting to break so I got some Sugru to mend the damage. Absolutely incredible product that saved me like $70 to get a new charger. The only downside is each package is not resealable. Luckily I also had an iPhone cable and Apple earphones to reinforce. So basically, Apple, get your s*** together and pack some Sugru with your products." —Leesankwok
Get an eight-pack from Amazon for $19.99 (available in seven sets of colors).