Last night I shared 200+ MBs of pics with a friend without having to upload / email anything. I just enabled the File Sharing in my Opera browser, set my folder and my friend was able to copy over everything directly!
Direct computer to computer file sharing, no restrictions or limitations, and the other person can use any web browser - you don't have to have Opera installed!
For my web browsing experience I still prefer Firefox, but this new feature of Opera - Opera Unite - has got me floored. They've brought the true power of the internet to the end user : It's not just file sharing; I can host photo sharing, chat rooms, games, ....
..file uploading, music/video streaming, my own websites - several things that have traditionally been server-side and for which you'd normally have to depend on 3rd party websites for (read: youtube, rapidshare, web hosters, etc). All the stuff I share is my own property here, no weird Terms and Conditions, no advertisements, no payments and no dependence on a server farm thousands of kms away.
It's platform-independent on the receiver's end : For example, if I want to share a folder full of files with you, I'll start the File sharing module of Opera Unite in my comp, copy the URL and share it with you. Then you can browse to this address from any web browser you may be using - for you it'll be like going to any other website. This in itself is a big leap from other peer-to-peer sharing software wherein both ends would have needed the same software installed. So imagine sharing your huge vacation photos (and videos!) collection with your parents and absolutely no technical headache on their part - just email them the link!
Also, there is no time wasted on your side either - you don't have to wait hours for all the stuff to upload and THEN share it like we normally have to on Youtube or Picasa or Flickr. So in big-picture terms, it cuts down absolute time taken by half!
They're still mint-fresh, and they've invited developers to use this platform to make more and more applications. I hope to see this grow and grow and also get adopted by mozilla soon!