How do I list all unique scripts in the block chain, and their count (n-times-used)?
If you are looking for actual input scripts then the answer is simple: if there are
If you are looking for output scripts then the number of distinct scripts is likely to be the number of addresses that received an incoming transaction, as they contain the addresses that may claim the output.
This is ofcourse just for the standard transactions in which you put up a claiming condition by requiring a single signature from an address and then provide the matching signature when claiming it.
More interesting could be the number of distinct script formats. That is counting the unique types of scripts, for example by just replacing all signatures and hashes with a placeholder and then counting them.
As far as I know blockchain.info does not currently offer such a functionality, but they might be interested to include it.