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What's great about drop.io is that all your files are stored on their cloud, so your recipients can see what files you've sent and either download them individually or all at once. You can send as many files as you want per up to 100MB per message.
From a development perspective, the Attach Large Files app is interesting because it was built entirely on top of the drop.io open API. The company wanted to show off the potential of the API and no better way to do that than build an easy-attachment app onto a huge webmail platform like Yahoo! Mail.
I love the idea of being able to send drops via e-mail with such ease and I hope other developers consider implementing this type of functionality into other platforms.