Extra Minds/Personalities

Alpha Centauri- Secrets of Creation, Unity of Spirit. Secrets of creation lets you make new personalities, Unity of Spirit lets you split your own
American McGee's Alice- Dreams and Imagination. Gives you an entire inhabited subconcious world.
Captain Simian and the Space Monkeys- Splitzy Personality. Makes a second personality, that's rather pleasant and calming to be around. Keeps you saneish
Changeling the Lost- Functional Split Personality. Splits you into Human and Changeling personalities, lets you switch at will
Dune- Abomination. Permanently fuses your mind with a dozens of other ones, drastically changing your personality
Eclipse Phase- Muse, Puppet Sock. Muse is a helper AI with a personality of your choice. Puppet Sock lets you download extra people into you, and switch between them
Exalted Infernals- Unwoven Coadjuster. Lets you keep the demon that exalted you. Can see what you see/think, and can offer advice. Upgraded one gets a spider sense
Exalted The lunars- Moonphase Aspects. Lets you split your mind, gaining 5 aspects of your choice to act as advisors
GURPS Dungeon Fantasy- Think Twice. Split your mind into two identical minds. Explicitely has access to all your powers
Gundam Anno Domini- Quantuum Brain, Super Soldier. Quantuum Brain gives you a second brain made of quantuum. Is a copy of your current brain, with all it's improvements. Super Soldier gives you a helpful Soldier personality
Kumo Des Ga- Parallel Minds. Not a perk, but a skill that can be purchased injump. Gives you extra minds, with the amount increasing as the level increases. The minds are their own beings, their personalities will deviate over time, and they're not guarunteed to be loyal to you
Majikoi- War and Peace. Gives you a pair of secondary personalities. Seiso, a calmer, wiser, and more compassionate personality. Haou, louder, boistrous, and punchy.
Psychonauts- Astral Layers. Makes every you you've ever been be it's own personality, to defend your mind
Tsukihime- The Killer in Me, The Moon's Shadow. Alternate personality companions. The Killer in Me is your murderous impulses. The Moon's Shadow is an ancient moon vampire.