An Identicon is a visual representation of a hash value, usually of an IP address, that serves to identify a user of a computer system as a form of avatar while protecting the user's privacy. The original Identicon was a 9-block graphic, and the representation has been extended to other graphic forms by third parties.
Don Park came up with the Identicon idea on January 18, 2007. In his words:
I originally came up with this idea to be used as an easy means of visually distinguishing multiple units of information, anything that can be reduced to bits. It's not just IPs but also people, places, and things. IMHO, too much of the web what we read are textual or numeric information which are not easy to distinguish at a glance when they are jumbled up together. So I think adding visual identifiers will make the user experience much more enjoyable.— Don Park, 
A similar method had previously been described by Adrian Perrig and Dawn Song in their 1999 publication on Hash visualization, which had already seen wide use such as in the random art of SSH keys.
- GitHub uses identicons to visually differentiate users who have not set their own avatar.
- Wikis and blogs may generate identicons to visually identify authors based on IP addresses. This provides some protection against impersonation without requiring authentication.
- Third-party software is available to generate identicons for the purposes of identifying eBay sellers.
- The original Identicon idea has been expanded to include a couple of new, simple yet very effective, anti-phishing protection schemes. One of them requires client-side support; Park is interested in talking to browser vendors regarding its incorporation. He calls this expansion "Gemini."
- There is an add-on for Firefox called IdentFavIcon Quantum that, on websites without favicons, replaces them in the tab with Identicons based on the IP address of the website.
- Anonymous email forwarding service IdBloc uses Identicons to help quickly distinguish between different email addresses visually
- Don Park. "Visual Security: 9-block IP Identification". Archived from the original on 2008-07-03. CS1 maint: discouraged parameter (link)
- Perrig, Adrian, and Dawn Song. "Hash visualization: A new technique to improve real-world security." International Workshop on Cryptographic Techniques and E-Commerce. Vol. 25. 1999.
- Munnin [ Product - Identicon ] Quick seller identification in the item listing
- "Identicon-based anti-phishing protection". Archived from the original on May 10, 2008. Retrieved 2007-02-15. CS1 maint: discouraged parameter (link) CS1 maint: bot: original URL status unknown (link)
- Don Park's original source code at GitHub
- Mavenized project based on Don Park's original source code at GitHub
- Identicons as Visual Fingerprints by Phil Haack
- Identicon CFC - ColdFusion implementation of Identicons
- NIdenticon - .Net implementation of Identicons
- Identicons used in the wild by IdBloc