Alex Brett

In addition to the gender markers "M" and "F", international standards allow for the marker "X" to be used in both human- and machine-readable sections of passports. Use of this option seems to be gathering momentum at the moment, and thanks to Australia's recent adoption public awareness is currently high. What that X actually means is, however, quite variable. Here's a summary of the situation as I understand it, presented in the form of a linkspam.

Australasia

• New Australian passports allow third gender option (BBC)
  
• The official policy (Australian government). Here X means "indeterminate/unspecified/intersex" and "[a] letter from a medical practitioner certifying that the person [...] [is] intersex and do not identify with the sex assigned to them at birth, is acceptable." To their credit the policy does use singular they - but overall this isn't much good for non-binary people
  
• Zoe has a nice clear summary of who will be affected and how
  
• New Zealand also offers gender X, but not for intersex people [content warning for degendering]

India

• In India the third gender option is displayed in the human-readable section of the passport as E (apparently for eunuch). I'm not clear on the usage or availability but I understand it is intended for use by hijra.

UK

• The UK Home Office is considering introducing gender-neutral passports (Pink News; no official sources yet)...
  
• ... and how best to implement this (analysis, Jane Fae).
  
• A discussion of the background can be found on Facebook
  
• Zoe suggests a wide-ranging consultation may be held
  
• Previously by Zoe: a Freedom of Information request about gender X markers

(Thanks to Nat at Practical Androgyny for ferretting out several of the UK-related links!)