Smartcards are not strictly speaking a derivative of PKI (many would argue that smartcards pre-date PKI), but we cover them here under derivatives for two reasons. First, PKI has become prevalent as an integrated part of most modern smartcards, including banking cards, national ID, employee ID, and health & welfare. And second, the attractive features of on-chip key generation and integrated digital signing services make smartcards an increasingly important key medium in PKI.

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SPKI (stands for "Simple PKI") was an effort, now defunct, to streamline traditional PKI.

The SPKI Working Group of the IETF worked in the late 1990s but was disbanded around 2001.

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WPKI is simply the deployment of public key infrastructure using private keys and cryptographic functions in wireless devices, especially mobile phones.

WPKI standards were developed by the WAP (Wireless Application Protocol) Forum, until that group merged with the Open Mobile Alliance (OMA).  

See also:

• WPKI Non Profit Association
• WPKI experience in Taiwan.
• The momentum for mobile

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The basic idea of PKI -- which can be thought of as a coordinated suite of technlogies, standards, management processes and agreements -- has led to numerous spin off approaches. Some like SPKI ("Simple PKI") have attempted to streamline the approach at the certificate protocol level. Others have deployed the basic elements in different form factors, such as wireless.

Historically the leading "IDtrust" technlogies and frameworks have been mainly based on Public Key Technologies (PKTs) and have been expressed as PKI.  There are however other fundamental mechanisms for delivering authentication.