As a designer, you probably work on more than one computer: one at work, one at home, a desktop, a laptop, iphone, etc. So how can you get all those files you use all the time somewhere that's easily accessible? Sure, we all tried dotMac and discovered how painfully slow it is. There's files emailed back and forth, flash drives and (gasp!) burning data to CDs or DVDs to access it on the go. Is there anything better? Yes, Dropbox.
Dropbox provides up to 3GB of free server space accessible with any web browser, a Mac-based desktop app and an iPhone application. Now, those files you were working on at home last night have a place to live where you can access them at work. You can cooperatively work on projects, share finished files with clients, store frequently used files and photos... pretty much everything you can think of and its all in a super easy-to-use interface.
You can upgrade to a pro account to access even more server space.
If your plan for 2010 was to get more organized, Dropbox is definitely a step in the right direction.