This article is from the X-Men Comic Books FAQ, by Kate the Short (email@example.com) with numerous contributions by others.
The original X-Factor members rejoined various incarnations of X-Men,
while the X-Factor kids generally became members of the New Mutants,
X-Force, or Generation X.
Alex Summers (Havok) was a member of X-Factor after the original X-Men
left. Havok presided over that team and was going to reorganize it after
their ties with the government were broken. Summers enlisted Shard,
Polaris, Multiple Man, Fixx and Greystone into his new X-Factor, only to
have the team go nowhere because Marvel used X-Factor #149 to ignite a
new series, MUTANT X. In the fateful issue, Greystone (who, like Fixx
and Shard, was a member of the XUE from Bishop's timeline) attempted
to return to his own timeline. The plan backfired, and the plane
carrying him and Havok exploded. Havok was sent to the Mutant X world,
inhabiting the body of that alternate universe's Alex Summers when he
"died." He appeared in the Mutant X title until it was cancelled. He was
seen apparently floating in Limbo after he "died" in Mutant X #32, but
he reappeared in a mental institution in UXM #411 and has since returned
to the mansion.
Lorna Dane (Polaris), who was also a leader of X-Factor, was on her own
for a while after Havok "died." Polaris later joined up with Magneto as
he ruled Genosha. During the Magneto: Dark Seduction mini-series, it was
revealed that Magneto was borrowing a portion of Lorna's powers in
exchange for lessons on how to use them. She turned against Magneto when
he attacked innocents to restore his powers with an enhancement machine.
She was seen helping the resistance in X-Men #112. After that, she came
back to join Havok, and seems to be quite a bit crazy after the events
of Uncanny X-Men (the 420s and 430s).
Jamie Madrox (Multiple Man) popped up on Muir Island with Wolfsbane,
Beast and Strong Guy. He was seen helping Forge in Genosha, though he
left as of X-Men #112. He reappeared as part of the X-Corps circa
UXM #401-404, and reappeared as a member of the X-Corporation in NXM
Guido Carosella (Strong Guy) had a heart attack. He later joined Lila
Cheney on yet another intergalactic tour.
Marshall Evans (Random) was last seen in Genosha, working with Blob
(X-Men #112). He was apparently killed in Weapon X #5.
Kyle Gibney (Wild Child) left X-Factor and was seen wandering about as he
mutated once more, ending up as a member of the new Weapon X title.
Pietro Maximoff (Quicksilver) starred in his own title for a while. He
helped Magneto rule Genosha for a short while. He left Magneto's side
during the Magneto: Dark Seduction mini-series. He appeared to be a member
of the Genoshan resistance movement until Avengers #38 showed that he had
rejoined his former teammates.
Shard, Bishop's sister, was kidnapped by Trevor Fitzroy, as revealed in
Bishop: The Last X-Men #2-3. Fitzroy was turning Shard human again. She
sacrificed herself in Bishop:TLXM #13 or 14 to give Bishop the energy he
needed to stop Fitzroy once and for all.
Forge showed up in X-Men Annual 2000, as well as in X-Men #102. He's
since joined Mystique in her new title.
Mystique became a terrorist and killed Moira MacTaggert on Muir Isle.
She was seen in the hospital in UXM #389, and reappeared in UXM #405.
In 2003, a Mystique title was launched, and she's been seen there and
in the Draco storyline circa UXM #432.