I generally carry:
Two water bottle holder thingies like these Weaver Clip-On Holsters w/Water Bottle, Water bottle holder, heavy duty, great for trail riding at TOHTC.com
But I don't use those bottles. It is generally a Polar bottle or even just the bottle the water came in! I put a hoof pick and my folding pink knife in the right side one and the left side has carried my camera, GPS, snack.
Cantle bag carries hoof boots, twine, toilet paper, small first aid kit.
I recently got one of these Cashel Trail Kit @ Adams Horse and Pet Supply
And used it for the first time this weekend. I put my compass and GPS in it. Camera, map, chapstick, the compass and GPS and Advil.
The next ride will include a little Magnalite Flashlight, extra batteries for GPS and snacks for me and The Biscuit. My GPS chucked out due to the batteries dying. Never again.