For example, using the bitcoin blockchain as a proof of existence mechanism[1] and using namecoin as a replacement for SSL[2].
What clever uses of cryptocurrency have you seen?
[1] http://www.proofofexistence.com/
[2] http://blog.mediocregopher.com/namecoind-ssl.html
pasted below: Alternative chain examples
1. Peer-to-peer, social trading of “normal” (government-backed) currencies without the need of a centralised clearing house. Called a ripple exchange, it is based on a design loosely inspired by the Ripple monetary system.
2. A decentralised, open DNS (Domain Name System) using the .bit Top Level Domain. This is an alternative DNS root based on Namecoin and outside the official DNS root administered by ICANN. Early work in this area included discussion on BitDNS.
3. A service called Proof of Existence which is a sort of notary public service on the Internet. It easily and cheaply (0.005 BTC) allows people to verify ownership, integrity, and that a document existed at a certain point in time.
Building on the Bitcoin protocol examples
1. Bitmessage for encrypted peer-to-peer communications, including hiding metadata like the identity of the sender and receiver, from eavesdroppers. The message transfer mechanism is similar to Bitcoin’s transaction and block transfer system, requiring a ‘proof of work’ for each message.
2. Zerocoin that augments the Bitcoin protocol to allow for fully anonymous currency transactions by placing anonymity technology into the Bitcoin network itself. Zerocoin uses Bitcoin as a “distributed, online, append-only transaction store.”
3. Gliph adds secure mobile communications to the Bitcoin landscape. It edges into controlling digital identity “that shares as many (or as few) facets of yourself as you want to.”