Originally Posted by Rowan

A few, but more for space than weight. I've trimmed off the handle end of a crescent wrench(shifting spanner) to fit in the bag, and I created a headset wrench from a thin cone spanner by grinding out the jaws and cutting off the original handle halfway.

Much comes down to building a bike so you have as few tools as possible. Aheadsets have virtually remedied the need for headset wrenches; you can use a piece of cinch strap looped around the cassette as a chainwhip, and most decent pedals these days have a hex hole on the spindle for removing from or attaching to cranks. Even the external BBs these days and "spindle" seet-ups are relatively simple -- although removal might require a cut-down wrench.

One of the important things about using "lightweight" or cutdown tools is to ensure that things aren't overtorqued. There is a predisposition by many to yank down on things so tight, that it is almost impossible to undo them again (and in many cases is impossible). Pedals and cassette lock rings come to mind. Grease all threads, and apply only the required amount of torque. If you don't know by feel, acquire a torque wrench for home use and find out; many of the specced torque readings on bike components are very light.

I remember the loads on my first days of touring included a 10" crescent wrench and various other bits of paraphenalia. Now it's down to one multitool (Topeak Hexus) and maybe a crescent wrench, a decent spoke key, and not much else. On randonnees it's just the Hexus.