Cheat sheet for weird and wonderful URI and URL formats
RFC 3986 defines the generic syntax and format for URIs and is a good primer on what's possible with URIs.
Uses the mailto: URI to signal the client to open a new e-mail message addressed to the recipient following the mailto: identifier in the default e-mail application.
<a href="mailto:nobody@example.com">Feedback</a>
Please read RFC 6068 for more information regarding advanced formatting.
Works similar to mailto:
<a href="tel:+31123456789">Call us</a>
Please read RFC 3966 for more information regarding advanced formatting.
SIP and SIPS URIs direct the client to make a SIP connection to a specific URN, over ports 5060 and 5061 respectivly, and (possibly) ask for a connection to a specific phone number.
<a href="sip:1-999-123-4567@voip-provider.example.net">Call us</a>
<a href="sips:1-999-123-4567@voip-provider.example.net">Call us</a>
Wikipedia has a short article regarding sip: URIs.
Also similar to mailto: but support is a lot rarer.
<a href="geo:37.786971,-122.399677;u=35">Wikimedia Headquarters</a>
Wikipedia has a short but sweet article regarding geo: URIs.
Similar to mailto:, opens a supported client for sending bitcoint to a wallet, if such a client exists.
<a href="bitcoin:3QJmV3qfvAf3sRCW3qSinyC">Send me Bitcoin!</a>