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 @+) |