I tend to keep everything of importance online in the first place these days -- Gmail has all my email so I don't have to think about Outlook migration... poo. I keep a number of frequently used documents in Google Documents (I created them online and don't even keep local copies). SourceForge has most of my interesting development work on it (and that's mirrored and backed up plenty). Some other miscellaneous data I keep on my own web site hosted with DreamHost. There are enough friends and family who want web hosting that I can have a DreamHost account almost for free because they pay for all their sites (which I also host for them), which pretty much pays for my hosting (I stop them before they pay for more than the cost of the hosting). Dreamhost has some really good deals.
What's left? Pictures. I haven't quite worked out the optimal way to store pictures yet. There are too many choices, and one hasn't stood out above the rest for me yet. Some pictures I manage with ... some picture management thingy in Ubuntu -- I forgot what it's called -- it lets me tag and categorize pictures, and keep originals and modified versions. Some pictures I upload to Facebook, some I upload to Picasa, some I upload to my own web hosting space in a simple folder. Some I use the program in Ubuntu to create a web site and copy that out to my hosting space. Most I keep in My Pictures in Windows. Yikes, pictures all over. I ought to pick a place and stick with it... Picasa looks good, but I think last time I tried to use the program in Ubuntu to publish pictures to it, it didn't work. Will have to try again, or just go directly to Picasa. Videos are on YouTube of course.
There's so much online that I hardly need to keep anything important locally, which is really convenient if/when I get a new computer or hard drive. I can use a new OS without worrying so much about migration of data. There are, however, still a number of files I don't keep online. I have some music compositions created in CakeWalk Express Gold, a few old documents that I don't know if I ever got online, and don't dare throw away (I suppose I could just upload those to Google documents), a music library of other music (hm, there may be an opportunity for someone to create a popular online music storage site). There are some miscellaneous downloads, which I could of course download again, and various program data files, which are not backed up anywhere. But I'm just realizing now that I have a remarkably small amount of important data locally. Mostly my music compositions I guess.
I think I like the direction this is headed.