Well, I don't think you'll find a free account with 20GB of space. 2GB is the most common free offer. I use dropbox.com from time to time to backup some stuff and share some other stuff. For $9.99/month you get a 50GB account, for free you get 2GB.
Megaupload and the like are file sharing sites, not personal backup ones (although you can use it like this if you are really patient, but it depends mostly on your files). For free you have the lowest download priority and bandwidth, limited downloading (daily limits, annoying "Download limit exceeded" messages), waiting before a download starts, no direct downloads, no access to files larger than 1024 megabyes, limited lifetime of the files in your account, and many more.
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