Perishable items would be interesting sometimes. Sadly, their one time use means you have to be sure that it'll guarantee you the win at that point.
Mainly things like Pinch Berries. Any sort of Sub/StatUpMove/PowerAttack/CoverageAttack @ Pinch Berry set only work once, especially since you're stacking the boosts from both your CM/SD/Agility/Claw Sharpen/Whatever and your Pinch Berry. But then, some go for raw power over coverage.
However, something that's really been making me think is Power Herb or White Herb.
Power Herb lets you skip the charge turn for Solarbeam and Sky Attack. I forget any other charge-up moves, but you know what I mean. Generally, things that benefit from it; Exeggutor's Solarbeam, Honchkrow's Sky Attack, Salamence's Fly. Then again, Life Orbed secondary STAB options do more damage...
White Herb on the other hand makes Leaf Storm and Overheat more viable on the likes of Defensive Celebi/Heatran respectively. But why drop Lefties? Lucario or Heracross can keep spamming Close Combat. But they'll die fast anyway...
I think the reason certain items aren't used are because they're just not as good or consistent as the top items; Choice, Life Orb, Leftovers.
It's hard to find a good alternative item for a Defensive Pokemon. Alternative Items for sweepers, Life Orb vs Expert Belt. Defensive Mons on the other hand, have to jump between Leftovers, Lum Berry, or even Shed Shell in the case of Steel-typed ones like Skarm or Forretress. But they're not that much better than Leftovers.
Pokemon is a game you play to win, and to do that, you need to have some of the best items on some of the best pokemon. Unfortunately, it just happens that the best items are very difficult to change, and there are only so many of them.