Redmine

http://stackoverflow.com/questions/658044/tick-symbol-in-html-xhtml
☐, ☑, ☒

RedCarpet

FYI this can be accomplished somewhat cleanly by creating macros in a plugin. This is the quick hack which I used, saved as plugins/redmine_wiki_checkboxes/init.rb:

require 'redmine'

Redmine::Plugin.register :redmine_wiki_checkboxes do
  name 'Redmine Wiki Checkboxes plugin'
  author 'Stephane Lavergne'
  description 'Adds x, z and o formatting macros'
  version '0.0.2'
  url 'https://github.com/vphantom'
  author_url 'https://github.com/vphantom'
  requires_redmine :version_or_higher => '3.4.0'
end

Redmine::WikiFormatting::Macros.register do
  desc "Unchecked checkbox" 
  macro :o do |obj, args|
    '<b style="font-size:133%;">&#x2610;</b>'.html_safe
  end
end

Redmine::WikiFormatting::Macros.register do
  desc "Checked checkbox" 
  macro :x do |obj, args|
    '<b style="font-size:133%;color:#009900;">&#x2611;</b>'.html_safe
  end
end

Redmine::WikiFormatting::Macros.register do
  desc "Crossed checkbox" 
  macro :z do |obj, args|
    '<b style="font-size:133%;color:#bb0000;">&#x2612;</b>'.html_safe
  end
end

They cannot be used as list bullets per se, but it still opens decent possibilities.

Also, having a crossed checkbox is not usual so I think we should not support this feature.