iOS: Receipts are ridiculously easy to lose, so I always try to snap a picture of important ones with my phone. From now on, I'll be taking those pictures with Piikki.
There are plenty of apps and methods to store receipts on your smartphone, but I have yet to encounter a faster option than Piikki. When you want to store a receipt, Piikki's built-in camera can automatically detect its edges and crop out the background, keeping the file size to a minimum. Once you've taken your photo, the app will automatically store a geotag with your receipt so you can remember where it came from, and you can also key in the amount to keep track of expenses.
My favourite feature of Piikki, though, is one-click uploading to Evernote, Google Drive or Dropbox. The receipt will still be stored within the app with all its accompanying metadata, but you'll know it's also safely backed up to your cloud services of choice. Piikki is a universal app available for $1.99 in the App Store.
Piikki: Receipt Scanner & Expense Reports [App Store]