Tabcomplete

You can use the TAB key in the Editbox (and in various popup dialogs like $? $input to complete various words)

See also Options -> Editbox for various tabcomplete options including disabling it.

TAB Completes and cycles all channel nicks, if current window is a query window, the nick of the query is used.
/ TAB Completes a /command including aliases from remote scripts/aliases.
nick TAB Completes channel nicks, query nick or your own nick.
$ TAB Evaluates a identifier, eg. $me TAB $chan TAB.
% TAB Evaluates a global variable.
# TAB Completes and cycles all joined channel names (and any channels in your Serverlist for the current server), if current window is a channel, the channel name is completed first.
#chan TAB Completes a joined channel name.
<@|%|+>nick TAB Completes channel nicks matching the channel prefix and/or nick. (Based on channel prefix received by the server, e.g @+)