(I know this is mostly about music, but you can use it to store non-Kindle ebooks online at Amazon).
Amazon has announced their
Cloud Drive and
Cloud Player services today. The first gives you 5GB to store anything (you legally own), including using it to back up your non-Kindle ebooks. I'll definitely keep using
Dropbox for most of my books, but have started building my MP3 collection there (purchases from Amazon can be stored on your drive automatically and don't count towards the 5GB limit, so I've repurchased much of free music that is still free). While doing so, I found a number of albums I hadn't seen before, including these:
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Allegro 2011 Winter Sampler free
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Music for Massage, Meditation & Yoga free
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Celtic to Classic - Relaxing Guitars free
You can also increase your storage to 20 GB by paying $20/year (larger amounts to 1 terabyte, at various rates) or, for the first year only, buy any album this year (non-free albums only) and get the fee waived. There are some interesting choices on the
100/$5: 100 Albums for $5 Each, such as
What's Going On, by Marvin Gaye, and
Greatest Hits - We Will Rock You Edition, by Queen, which would let you buy an album and save $15 off the extra storage charge.