iCloud

Apple's just announced  multimedia cloud storage thingy, iCloud[1] looks like a real threat to Dropbox.
Especially if well integrated into the Apple ecosystem (Mac, iOS and applications).

Not for me, though; the 2GB Dropbox offers for free are sufficient for me... but thats only because I do not keep music or photos/videos there..

Also Dropbox will probably always be more cross-platform.

So I'll keep using Dropbox - and invite you to do the same.

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[1]  if they can keep the name :-)