Light stands, umbrellas, tripod, etc., always in shoulder-cases in the car. Don't mind leaving them there on the floor, car locked, if I don't need them.
More extensive shoots? Kata 3N1-22 with pretty much everything I think I might use. Loaded, it's 21 pounds. Very comfy as quick-access sling or backpack.
Grab-n-go everyday? Kata H-16. LOVE this bag! 40D (Crumpler Industry Disgrace strap) with lens and hood, two more lenses w/hoods, 580 EXII, wireless remote release, 3' ETTL cord, filters, gels / bounce cards / flag/snoot (flat in rear pocket), spare batteries, CF cards, microfibre cloths, its own raincoat if weather looks iffy.
Sling it outta the way behind my back, or on my side with arm and hand draped over it in crowds, bring it around front to grab camera, get flash, switch lenses.
Bag's compact and perfectly shaped / designed, for me. Light even always loaded and ready. Never leave home without it.
If I think I'll need it, I'll throw a 430 EXII and long ETTL cord in jacket pockets or Big Pockets vest.