Although nearly everyone I play with swears by the Ring of Sustenance, I end up almost never using one. Not sure why, it's not like I play a lot of arcane casters.
Items I do use:
Heward's Handy Haversack: I don't think I've ever created a character past about 5th level or so that didn't spring for one of these, and few characters get played that far without at least trying to acquire one. It's just so darn useful for toting stuff you might actually need. Sure, a bad of holding is better for hauling loot, but the haversack is where you stick all your spare potions, oddball tools, extra rations, rope, odd bits of treasure, extra waterskin, bedroll, backup weapons, etc.
Quiver of Ehlonna: Similar to the Haversack, though I rarely use this until fairly high level. Forget the obvious use for archers, where else does an archmage store his extra wands, staves, metamagic rods and the like?
Various mobility enhancing items. Boots of Springing and Striding, Wings of Flying, Ring of Jump, all those sorts of things. Mobility is a great asset on the battlefield, and can be darn useful for avoiding battle in the first place, not to mention circumventing traps and the like.