Set of allens
Set of ring spanners
Stick of epoxy
Little double action bike pump
Bowden cable emergency repair kit
No idea what it weighs but it's nice and central so I don't really care. :-)
Though I've been considering building a new under seat tray of some kind, given that I've ditched the old 25Ah car battery for a PC680 mounted across the frame, and now have a gaping hole there that can be filled with Stuff.
It'd be nice if I could get the old Cyclelok six arm in there so I could lose the top box.