Amulet of Wizardry, Helm of Wizardry, the wizard's own upgradeable staff, Skeeve's set, wizards can get a lot of extra spell slots. Sorcerers get many extra spells also, having available a Helm of Sorcery, Skeeve's set, the upgradeable sorcerer's staff, missing out only on an amulet. Bards are not so lucky, with Tune Master gloves available at higher levels and a certain longsword at MUCH higher levels.
I haven't completed the Jester's set (high bard) to know if it gives bonus spells to the bard. With Pugz' help, I WAS able to complete the Blessed Set, which I had thought was supposed to be the high level cleric set.
It turn out the Blessed Set does NOT give bonus spells like other caster sets do. In fact, to complete it, you have to remove the single item in the game that DOES give bonus cleric spells, the humble cloak. Also have to remove your other item that increases wisdom, the amulet, resulting in a further loss of memorized spells.

It would be nice to see a few more stand-alone items with cleric bonus spell slots, available at different levels. A ring of holiness (normal, greater, epic) is just one example. On another note, I was trying to think of something that would go up in level similar to the arcane caster's personal spell staves. As a cleric levels up, growing in power, he should be able to channel more power through his holy symbol, such as a cross, displayed on armor, shield, or tabard (cloak, in NWN). The holy symbol could be bumped up in power similar to the caster's staves, perhaps by praying at the altars?

That might present some problems for a melee combatant in our world, though. I thought armor at first, then realized it couldn't be removed to put resistances on it, and might prove to be a liability eventually, same deal with a shield. Using a cloak to hold a holy symbol makes it so that the cleric can't use the one item in the game that already gives bonus spells, but other casters already have this issue with their silver/gold/platinum spell staffs (but they have other sources!)