Dropbox (file storage and synchronizing service) uses a lot of hashing behind the curtains. For example if you try to put huge but common file in your Dropbox (large patch or whatever) its hash is calculated. If file is already present on servers it is linked to your account. Without wasting time and bandwidth.
To be clear, this means that if I upload a 10 meg pdf to my space, Dropbox creates a unique id from that file and stores it in a database. If you later upload the same file, Dropbox is smart enough to just copy my pdf to your folder, saving you (and them) a ton of bandwidth.
Discovered via this AskMe question, where a user noticed that ~600MB of files uploaded almost instantly.