I know of the spell, but I don't like the duration. I'll still use it, of course, but it's not the best.

What items grant this ability? Any ways besides items or spells?

Of the top of my head:

Amulet of Power (from Aran Linvail)
Improved Mace of Disruption (MoD + Illithium)
Spectral Brand +5 (Cespenar makes)

You can use a barbarian or berserker's rage ability. Don't know and don't think Minsc's rage gives this too.
Of course (I presume you know this already) an undead hunter is also immune to it.
Slayer transform is also immune IIRC but you may not have access to it yet.
Concerning spells I prefer to use limited wish.

For the items, there is a very complete item guide on SP (does not include mods) by Dan Simpson.

Angurvadal +5 and Runehammer +5 also makes you immune to level drain. Cespenar makes these for you.

Minsc's rage seems to be identical to a berserker's, though occasionally you can lose control of him (doesn't always happen). If that's true, then it makes him immune to level drain.

Lastly, the Cleric of Lathander's innate, Boon of Lathander iirc.

*Quote*
Minsc's rage seems to be identical to a berserker's, though occasionally you can lose control of him (doesn't always happen). If that's true, then it makes him immune to level drain.

I might be wrong or misinformed, but I dont believe Minsc' rage is identical to the berserker's rage.
Given the information that Rage makes said berserker immune to imprisonment spells, I sent in Minsc to the Beholder City (BGII) and Raged him. He lasted almost 2 minutes before an Elder Orb imprisoned him, much to my dismay.

Needless to say, I have to be somewhat sceptical of his being immune to level drain.

Funny, Minsc's berserk always worked fine to stop him from getting imprisoned by Kangaxx. That's why I was mentioning the various things with Minsc's berserk, because it seems kind of screwy.

One of these days I'll stop being lazy and look at it with a modding tool.