Aberration-Based PrC/Spells?

[CockroachTeaParty]

Lords of Madness would be your best go-to source for such things.

Prestige Classes from that book mostly concern fighting aberrations, but there's a PRC called the Fleshwarper which specializes in grafting things to their bodies; I think in the end they actually become aberrations (I know their familiars do). It's really hard to get in to, though.

Said book contains a number of Aberrant Feats that give your character unsettling, aberration-y things. Extendable limbs, webbed feat, compound eyes, etc.

There's also some aberration-flavored spells; Undulant Innards is one, I believe, that makes your internal organs move around and makes you less vulnerable to precision damage and critical hits.

The Book of Vile Darkness might also have some things of use; a few Beholder-related spells, if I'm not mistaken. *belch*