Android: Google removed the filter at Google Play a while back that let you see the apps you've purchased. Now, the ones you've paid for are shown along with every other app you've ever downloaded, even if that app is free or no longer on your phone. My Purchases is a handy Android app that shows you everything you've paid for at Google Play and how much it cost you.
My Purchases doesn't just stop with apps either. If you've ever purchased books or movies through Google Play, they will show up in the list as well, all colour coded so you can quickly tell them apart. Each item is also shown with its sale price next to it, so you know how much you spent on the item. The app even supports multiple Google Accounts, so you can see apps you've purchased for work versus ones you purchased for fun, for example. My Purchases is free and ad-supported. $1.29 will get you a "pro" version that strips out the ads.