Technically a add on is called a modification or (MOD).
They are not supported here by the vendor.
You may try UBBDEV they are a group of people mostly visitors here that provide mods.
But to answer your question for new posts you have a few options in the stock software already.
Active topics: Menu at the top of the screen.
Watched topics: Accessible from the My Stuff menu at the top of the screen.
Post islands: Here it is called recent topics that can be configured from the control panel.
