Wired has the breakdown in this month's What's Inside. For starters, lots of water:
The ink in inkjet cartridges can be as much as 95 percent superpure deionized water. Yet at more than $US3 per milliliter, it would be cheaper to print your vacation pics with Dom Perignon.
The rest of the ingredients happen to be things like anti-curling agents, adhesives, thickeners, anti-evaporators and, well, dyes. [Wired]