You can do a search and find a topic on this... Pretty in depth where everyone posted the stuff they carry with them.
I haven't gone on any really long or extended rides for more then a day or in the desert or remote mountains, etc., so for me at this point I carry:
1) Simple tire repair kit. The kind that you push a "plug" into the hole. Comes with glue for the plug, etc. These kits work great and I've used them a LOT. (Mandatory)
2) A mini air compressor to fill up the tire. It's a Airman Sparrow I think and fits perfectly under the seat in the very back. (Mandatory). Note: Some people like the CO2 cartridges to fill up the tire, but I've heard they don't always work, and if you run out of cartridges (waste them, whatever) you're screwed. I want something reliable that will WORK when I need it to so I prefer a small air compressor.
3) Stock tool kit, plus a Leatherman tool. (Mandatory)
4) Air gauge, registration, manual, proof of insurance. All this stuff 1-4 is under my seat which I like because they are my "critical" items.
5) Cell phone, quarters, money, garage door opener, lighter, various other crap in my tank bag.
6) In my rear tail bag, I keep extra gloves, ear plugs, maps, bungy cords, rain covers for the bags, my outer jacket liner, water bottle, and various other junk.
That's it! I travel light.