Really hidden features

<ctrl><space> will start autocompletion even on an empty string
if there is an autocompletion popup, <tab> will do autocompletion similar to command-line-completion
you can change shortcut-key combinations, simply move your mouse over a menu item and press a key combination to set it
double-click in the margin to add a bookmark
move your mouse over highlighted syntax to get reference information if available (currently PHP functions are the best examples)
right click on a document tab to see a listing of all opened documents
<ctrl>mousewheel will change up/down the font size (a zoom like effect)

GTK hidden features

<shift><control>u: Unicode input: type the four numbers of the hex code and finish with Enter.