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A friend has asked me if any such thing exists, I have no idea, anybody here use any?

What does your friend want it for? If they want to learn animation in an effort to break into the industry, then the major animation software vendors all have free trials:

Houdini Apprentice limits the output image format to a proprietary image format -- conversion/output to other formats applies a watermark. Available for Windows or Linux. Personally, I'd start with this one over the others, since it also includes a full compositing package.

Maya Personal Learning Edition has a more obtrusive watermark. Available for Windows, Linux, and OS X, though I've heard the OS X version isn't that great.

XSI has a free trial, as well, though I don't know if it's renewable, the way Apprentice is. I'm not sure of the OS support on this one.

3ds Max has a 30 day trial, but again, I don't know full details. This one is Windows only, however.

Those may be appropriate for some small personal projects as well (all of them have licenses restricting use to non-commercial projects), depending on whether or not the watermarks are an acceptable tradeoff.

The only truly free option worth considering is, as mentioned previously, Blender. There are a number of low-cost animation packages, but I honestly wouldn't bother with them given the availability of the above software, and Blender.