How to obtain the complete list of available Drush Output Formats?
To see the verbose list of output formats use help with a command name. For example the help for the watchdog list command drush help wd-list:

Normally you get:

 --format=&lt;json&gt;                           Select output format. Available: table, csv, html, json, list, var_export, yaml.
                                           Default is table.


But if you use --verbose:

drush --verbose help wd-list


You get the full list:

 --format=&lt;json&gt;                           Select output format.
                                           All available values are:
                                            - table: A formatted, word-wrapped table.
                                            - config: A configuration file in executable php format. The variable name is
                                           "config", and the variable keys are taken from the output data array's keys.
                                            - csv: A list of values, one per row, each of which is a comma-separated list of
                                           values.
                                            - html: An HTML representation
                                            - json: Javascript Object Notation.
                                            - labeled-export: A list of php exports, labeled with a name.
                                            - list: A simple list of values.
                                            - php: A serialized php string.
                                            - print-r: Output via php print_r function.
                                            - var_export: An array in executable php format.
                                            - variables: A list of php variable assignments.
                                            - yaml: Yaml output format.
                                           Default is table.


To get a global detailed list of all formats defined in drush you can use the drush function outputformat_drush_engine_outputformat():

drush ev 'var_dump(outputformat_drush_engine_outputformat())'


EDIT:

If you want to see only the format names you can use:

drush ev 'print_r(array_keys(outputformat_drush_engine_outputformat()))'


Output:

[0] =&gt; table
[1] =&gt; key-value
[2] =&gt; key-value-list
[3] =&gt; json
[4] =&gt; string
[5] =&gt; message
[6] =&gt; print-r
[7] =&gt; var_export
[8] =&gt; yaml
[9] =&gt; php
[10] =&gt; config
[11] =&gt; list
[12] =&gt; nested-csv
[13] =&gt; csv-or-string
[14] =&gt; csv
[15] =&gt; variables
[16] =&gt; labeled-export
[17] =&gt; html