AutoComplete.txt

http://www.sublimetext.com/docs/3/auto_complete.html
http://docs.sublimetext.info/en/latest/reference/completions.html
Type a few characters → Auto complete list popup as you type → Tab or Enter to choose
Force open by Ctrl+spacebar
Ctrl+spacebar to select next entry
Enabled by default for source code and HTML (but only after entering a < character).
Autocomplete list comes from (priority from highest to lowest):
    .sublime-snippet  [[Snippet.txt]]
        only fire if exact match of trigger
        Show up in the completions list in the pattern: <tab_trigger> : <description>
    API-injected completions, i.e: EventListener.on_query_completions()
    .sublime-completions
        scope  =  Determines whether the completions are to be sourced from this file
        completions  =  Array of completions
        trigger = Text displayed in the completions list. When select will insert contents
        contents = Text to be inserted. Can use snippet features.
        Example:
            {
               "scope": "text.html - source - meta.tag, punctuation.definition.tag.begin",
               "completions":
               [
                "attrstamp",                                           //trigger and content is the same
                { "trigger": "acronym", "contents": "<acronym>$0</acronym>" },   //acronym → <acronym></acronym>  with the cursor placed in the $0 position
                { "trigger": "assert", "contents": "assert ${1:condition}, ${2:message}" },  //mimic Snippet syntax [[Snippet.txt]]
               ]
            }
    Words in the buffer
Can specify condition to ignore autocomplete via [[tmPreferences.txt#cancelCompletion]]

Menu bar > Preferences > Settings:
    "tab_completion": true,               //Shift+Tab/Shift+enter to ignore Autocompletion
    "auto_complete": true,                // Enable auto complete to be triggered automatically when typing.
    "auto_complete_size_limit": 4194304,    // The maximum file size where auto complete will be automatically triggered.
    "auto_complete_delay": 50,            // The delay, in ms, before the auto complete window is shown after typing
    "auto_complete_selector": "source - comment",     // Controls what scopes auto complete will be triggered in
    "auto_complete_triggers": [ {"selector": "text.html", "characters": "<"} ],     // Additional situations to trigger auto complete
    "auto_complete_commit_on_tab": false,    // By default, auto complete will commit the current completion on enter.
    "auto_complete_with_fields": false,     // Controls if auto complete is shown when snippet fields are active.
    "word_separators": "./\\()\"'-:,.;<>~!@#$%^&*|+=[]{}`~?",  // Characters to separate words and not trigger Autocomplete