Every time (unless I forget):
- mini-pump/CO2 inflator (single combo unit)
- 16g CO2 cartridge
- glueless patches
- extra tube
- tires levers

Optional:
- more tubes
- more CO2
- real patch kit

The glueless patches are handy for quick patchings and for small tears in a tire. Bigger tears can be dealt with on the road via a "boot", either an official commercial "boot" (e.g. by Park Tools) or an improvised boot such as a folded dollar bill or scrap of Powerbar wrapper (the mylar is good stuff). The nice thing about a CO2/minpump combo is that it's easy to prime the tube with the minipump to give the tube some shape for mounting on the rim, and, perhaps more importantly, you can be pretty sure of a good seal before blowing a cartridge on it.

Heh, in writing this it occurs to me that my 20" folder tires may not need a full 16g cartridge (it's perfect for the roadie). Hmmm...

Edit: speaking of my folder, with the rear wheel not having a quick release (consequence of the hub gearing) I also take a Park Tools "road wrench" 15mm wrench. Very handy: it's meant to be a pedal wrench, I suppose, and I use it as such as well.