This is a very open/general question (I hope not too general anyway:))
I'm looking for a library/module that could be plugged in a web server (like apache) and handle REST requests to store / retrieve / delete files. Something like Amazon's S3 or Windows Azure storage, but open-sourced.
Does such a thing exist?
mod_dav? DAV is the original generic/bare-bones REST. You PUT files, then you can GET them back or DELETE them... But that doesn't provide any management by itself, and maybe that is that you are looking for. Have you looked into OpenStack, specifically the object storage component?
There is OpenStack SWIFT which is open source clone of the Amazon's s3. It is lineary scalable and provides REST interface to the data. http://swift.openstack.org/
I solved a similar problem using Node-FSAPI, a NodeJS-based server that exposes a selected part of the file system as a REST api. (It's not an Apache module like you asked for, but it solves the same problem.)
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