I have a 13 button mouse (Logitech G700). I use the left, middle and right button as normal. The wheel tilt left and right I have mapped to open a new tab and close a tab in a browser. The close tab is Ctrl+w which closes a lot of things, at least in linux. I use the ?zoom? button to refresh (F5). The three buttons beside the left button I have mapped to raise, lower and mute the system sound. The four buttons on the side I use to navigate forward and backwards, and to go to the next or previous tab.
My mouse buttons are obviously used mostly for web browsing. I use i3 for a window manager and do most of my work in emacs and on the command line where using a mouse seriously shows me down. I don't use my programmable buttons much because I have to lift my hands to get to them. I do have my left control and caps lock swapped though.