Strange, that no one mentions Wuala.
I've been using it on my computers for a while. It's written in Java, multi-platform. You get 1gb for free.
However, you can "trade" storage on your own PC. (Or buy additional space, which is really cheap.)
Trade guide, from their website:
Wuala allows you to trade idle local disk space on your computer for
online storage. As an example, you could trade 100 GB on your computer
and get up to 100 GB of additional online storage in return. You get
whatever you provide on your computer multiplied by your online time.
In the example above, you would get 70 GB of additional online storage
if you're online 70% of the time. Clearly, this doesn't give you extra
storage, but it changes the quality of your storage - you can access
your files from anywhere, make an off-site backup, and easily share
files among friends and in groups.
There is only one requirement you have to fulfill in order to be
able to trade storage:
- You need to be online for at least 4 hours a day.