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Ideally, I am looking for a service like http://statoshi.info/, but one that displays the real-time number of inputs and outputs in the blockchain.
Note that I am not looking for information on the UTXO set, but the set of all inputs and outputs that are in the main bitcoin blockchain.
This is a useful statistic for developers because it has implications for the data storage requirements of parsing the blockchain and storing data per input/output.

Would you want just the current number, or also a historical graph? – Nate Eldredge – 2015-07-31T14:53:26.923
@NateEldredge The current number would help. A historical graph isn't necessary, as that information is out of date. However, those current numbers, while helpful, wouldn't really give an answer to the question, as I'm looking for a service/API that displays the realtime number, so that this question can be useful to others looking for this data whenever they find this question (I don't want the answer to go out of date quickly). – morsecoder – 2015-07-31T15:20:49.307