Members who have never been Mutants
Madelyne Pryor - Possessed no powers, served as technical support.
Associated with the X-men as she is a clone of Jean Grey
Longshot - A manufactured humanoid action celebrity from Mojoworld.
Fantomex - Escaped sentinal experiment from the Weapon Plus Program
Mimic - Dad's potion allowed him to mimic mutants powers.
Hepzibah - Extraterrestrial
Lockheed - Alien Dragon from Kitty Pryde's fairy-tale bedtime story. 'Nuff said.
Juggernaut - Ex-champion of Cyttorak
Omega Sentinel - Cyborg sentinel
Bamfs - Not Actually Bamfs. Non-mutants from an alternate dimension.
Sharon Friedlander - Enhanced by a Demon Bear. Acted as a nurse, joined Moira’s X-men.
Tom Corsi - Another Demon Bear victim/member of Moiras X-men.
Charlotte Jones - Detective member, given a suit by Forge. Warren's Girlfriend for a while.
Moira MacTaggert - Maid, Xavier's lover, M16 user, genius.
Members who did not see field work
Stevie Hunter - Kitty Pryde's dance teacher and physiotherapist to the team.
Members that temporarily lost their mutanthood
Storm - Went 41 issues in the mid-'80s without powers, even leading the team!
Members that permanently lost their mutanthood
Polaris - Lost her powers and X-genes on M-Day, regained facsimile powers via spinal implant
Magneto - Lost his powers and X-genes on M-Day, regained facsimile powers via special armour.
1. Names taken from here, and here other details from wiki/memory.