15 Function Mini Folding Tool

This multi-tool from M-Wave is not only incredibly useful but will fit in the palm of a hand.  Throw it in a pocket or bag to carry with you for adjustments on the fly.
15 functions in a convenient package
2/2.5/3/4/5/6/8 mm hex bits
2 Phillips and 1 flat screwdriver heads
T25 Torx and 3.2/3.3/3.5 Mavic spoke adjusters
Chain riveting tool