This article is from the X-Men Comic Books FAQ, by Kate the Short (firstname.lastname@example.org) with numerous contributions by others.
Calvin Rankin (Mimic) was seen in X-Force and Excalibur. Most recently,
he was part of the new Broterhood of Mutants in the High Evolutionary
storyline (circa X-Men #99 and UXM #379).
John Proudstar (Thunderbird I) was killed in the line of duty in UXM
Alison Blaire (Dazzler) left with Longshot. After staying with the X-
Babies in Mojoworld, she returned to Earth and then returned to the
Mojoverse (X-Men #113). Her life story was told in X-Men Unlimited #32.
She showed up again in Deadpool #67.
Longshot left the X-Men to find himself, reappeared, left to deal with
Mojoworld, and was last seen with a group of former mental patients
roaming the midwestern United States on their way to New York City. This
took place in the Longshot one-shot. He showed up again in Exiles #18-19.
Sarah (Marrow) left the X-Men powerless, and served as a camp counselor.
The Spider-Man/Marrow one-shot revealed that she'd been captured by
SHIELD and reprogrammed as a sleeper agent who would return to her
mutant form to hunt a target. Unsurprisingly, this created a serious
mental imbalance. She was thought dead, though the issue revealed that
she actually survived. She later appeared as a member of the Weapon X
Maggott left the X-Men in search of better training. He showed up in one
issue of Generation X and left to pursue an artifacts hunter. He was
apparently killed in Weapon X #5.
Cecelia Reyes left the team to focus on her career as a doctor. In X-Men
#80, Reyes was still associated with the X-Men despite having set up
practice in Salem Center. She reappeared when a wounded Nightcrawler
showed up in her inner-city clinic. During a fight with the Neo, Reyes
became hooked on a power-boosting drug. The X-Men helped her get clean,
and she has since left the team, probably to return to her practice.
Betsy Braddock (Psylocke) died in X-Treme X-Men #2. Her death was
confirmed in XXM #3.
Sharon Friedlander died during the Fatal Attractions storyline.
Tom Corsi was last seen as a Generation X gym teacher circa #69.
Stevie Hunter last appeared in X-Men #30.
Spiral was last seen in the X-Babies Reborn one-shot.
Destiny died at Legion's hands during UXM #255, in the Muir Island saga.
Destiny was last seen as a ghost in Limbo (with Legion and Margali
Szardos) in Fantastic Four vol.2 #16.
Stonewall died in UXM #255, during the Muir Island X-men stories.
SuperSabre was decapitated New Mutants Annual #7, in one of the Kings
of Pain Annual back up stories.
Crimson Commando was badly injured in NM Ann #7, but he came back as a
partially nuts semi-cyborg. He later showed up in X-Men Annual #2 and
Post died in Cable #87 when Pyro prevented him from killing Senator
Pyro died in Cable #87 after stopping the Brotherhood of Mutants from
killing Senator Kelly.
Senator Robert Kelly was assassinated by a human who was mad that the
Senator had changed his platform away from mutant-hating towards
compassion (X-Men #108).
Bastion was last seen in the M-Tech Warlock issues.
Fitzroy was teleported into bits in Bishop:TLXM #15. Bantam apparently
was left behind.
The Neo were recruited by Magneto in X-Men #110, after he killed two
of them. We haven't seen them since then.
The Reavers and the Shadow King last appeared in X-Treme X-Men Annual
2001. Pierce and some Reavers reappeared in New Mutants II #5.
Avalanche, Blob, and Radius all appeared as members of Banshee's
X-Corps in UXM #401. Prior to that, Avalanche was last seen in Cable
#87, Blob was last in X-Men #112, and Radius was last seen in Alpha
Flight volume 2.
Lady Mastermind (one of two feuding half-sisters) appeared in UXM #405.
Lady Mastermind (the other one) appeared in X-Treme X-Men #9, working
with Sebastian Shaw. Shaw popped up again in NXM #142.
Empath last reappeared in X-Treme X-Men #31.
Assorted Weapon X types, Alpha Flight members, and X-Men villians have
shown up in the Weapon X series, including Marrow, Kane, Sauron, Mesmero,
Maddison Jeffries (Box), Sabretooth, Wildchild, Agent 0, Wildside, Reaper
and newby Washout. Those guys formed the Weapon X team, with Soldier X
(Cable), Domino, Meltdown, Maverick and Blaquesmith appearing as the
opposition militia. Among random faces have been Locus of the MLF, whose
death by by Sabretooth was revealed in #2, and Omega Red, who showed up
in #3. Diamond Lil and Reaper were both killed in #5.
Other Alpha Flight members have shown up in issues of UXM, including
UXM #422 and again in #432.
Sabretooth, always busy, appeared in Deadpool #59, where he killed
Copycat, and also appeared in NXM #142.
(Mr.) Sinister showed up in an issue of Unlimited, where he offered to
resurrect Colossus, and was featured in Weapon X #13.