RentMy.World is a new take on an existing concept.
You need something just-this-once but it's too expensive to buy. Where do you go? ebay for a second hand one? That takes time, and you never know what you're going to get, or even if it's cheaper. The free sites? You should be so lucky. So why not rent one? Professional level tools have been hired out for years, but now you can rent anything - as long as someone has it to rent!
What about the things you've got lying around? That ladder you bought when you needed to clean the gutters, but sits around doing nothing the rest of the year. The spare power supply you bought when you thought the one in the computer had blown, before you realised it was actually a power cut. The spare hoover sat in the garage because someone just needed the latest Dyson. Make them pay for themselves!