[rocqua]

There is the option of converting to DNG files. Which allow for really good lossless compression. This does come at the cost of changing the file format, and risks losing metadata. That's why I personally decided to just buy more storage instead.

Come to think of it, have you tried running a modern compression algorithm on the data? I don't think I did. Could be cool if combined with ZFS or similar to get the compression done transparently.

[aspyct]

Converting the CR2 (Canon) to DNG tends to double or even triple it's size, but I haven't tried compressing it afterwards.

I should, as you suggest, test a more exhaustive list of formats, who knows...