Park Tools patch kit
[img]http://www.cambriabike.com/Images/product/park_gp-2_super_patch_kit.jpg[/img]
I used to carry two to three tubes (the fear of the "bad day" in terms of punctures) but now it's just the one with one of these minuscule pre-glued patch kit, which patches are good enough for mending on the fly glass-induced punctures, which seem to be 99% of what I get on the road especially in the winter.
Rear-view mirror
[img]http://gallery.rei.com/media/705468_9996Prd.jpg[/img]
Don't laugh! I tried one this week and am definitely leaving it on - don't know how I did without one now. Makes commuting ALOT safer and takes out the guess-work, I find myself looking into it all the time. At night, a quick glance is all it takes to know what's coming up behind you.
Ergon bar grips
[img]http://images.rei.com/media/274162.jpg[/img]
Yeah I know, I was ready to give those out a while back without having tried them. Well now I have and am leaving them on (flat bars). The position is sooo comfortable, and they allow you to pull/push on the bars, a bit like bar-ends.
Oh and lots of batteries for rear lights!