Sugru is an air-curing rubber that can be formed and shaped by hand, sticks to most surfaces, cures to a tough flexible silicon overnight, and is stable at -60°C to + 180°C. Strong, flexible, waterproof and durable, it can be used for a multitude of purposes to adapt, modify and repair. Its uses are really only limited by your imagination. Once cured it can be removed with a craft knife and the residue removed with fingernails and tissue paper if required.
Hard to explain all the uses so have a look at the clip after the jump, or visit the Sugru website. Available in multi-application packs from £7.50 ( $12 ).
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I like Sugru, but I also like some other variants on it that may be helpful to the maker, like epoxy putty, oogoo (http://www.instructables.com/id/How-To-Make-Your-Own-Sugru-Substitute/ ), Kwik-Plastic (http://www.farmandfleet.com/products/691417-kwikplastic_repair_epoxy_putty_stick.html#.TxeVR287WAg ), and so on.
Sugru IS handy, tough, and works great for many things. It does have a fairly short shelf life, and the fairly short working time makes it difficult to use for larger projects, but it is pretty cool stuff!