The input options toolbar is located directly below the input bar textbox.
Opens up a bar showing all the smilies that an account has set. Clicking one of them closes the bar, and adds the text for the smilie into the input bar textbox.
Opens up a bar showing the 15 colors that mIRC Color Codes allows. Clicking on one color makes the foreground (text) color change to the color that was chosen.
Translates text to another language for the user. It can either change the language of the output (in the chat pane) or of the input (from the input bar textbox)
Mibbit includes a basic pastebin service for those who need it. It tries to do syntax highlighting, even with pastes that are not code. Results vary wildly with it. When done, it puts a link into the input bar textbox.
Mibbit also includes a basic file upload mechanism. There is a limit of 2MB for the file, though it can be of any filetype. When done, it puts a link into the input bar textbox.
Need a long url to be only a few characters? Mibbit also allows a quick version for this too. Put in a link, and it'll put in a (hopefully) shorter URL in the input bar textbox.