I have an object which I would like to address using different keys without actually copying the object itself, like a symlink in Linux. Does Amazon S3 provide such a thing?
Symlinks no, but same object to multiple keys, maybe.
Please refer to Rodrigo's answer at Amazon S3 - Multiple keys to one object
If you're using the website serving on S3, you can do it via header x-amz-website-redirect-location
If you're not using the website serving, you can create your custom header (x-amz-meta-KeyAlias) and handle it manually.