First of all, let me preface this by mentioning this blog post from Nanto from the Skaro Hunting Society.
The link above is basically a huge post on most of the currently active groups doing older anime series. It's a pretty exhaustive list. However, the slant of that list tends to be slanted fairly heavily towards the shounen/mecha side. Because of that, Nanto missed out on quite a few groups, the vast majority of which are the groups working on mostly shoujo or kids series.
Let me also state that most of the groups working on old school anime are very small groups. There simply aren't that many people interested in working on series of this vintage. My definition of old school anime is anything before 2000.
So, without further ado, let me begin listing groups, in no particular order....
Silver Zero Subs - http://silverzerosubs.blogspot.com
Older series that they are working on: Heidi - Girl of the Alps, Akage no Anne
This group is working on two of the most loved World Masterpiece Theater series out there. Their releases are done with subs in English and Portuguese. Overall, it's a wonderful group doing a really good job with their work.
Takara Fansubs - http://takara-fansubs.blogspot.com
Older series that they are working on: Ashita no Joe 2, Homeless Girl Remi, Romeo's Blue Skies, Homeless Boy Remi, Takarajima
This group is one of my favorite groups that I've been following. I admit that the only thing I've watched from them is Homeless Girl Remi, but the work they've done on that is nothing less than superb.
Saitei Fansubs - http://www.saiteisubs.com
Older series that they are working on: Tokimeki Tonight
Tokimeki Tonight is probably going to be this group's crown jewel that will really establish their reputation as a old school anime fansubber. The series is so funny and the job they've done is so wonderful on it that future generations will laugh for years to come.
Nightvision - http://nightvision-web.org
Older series that they are working on: Lady Georgie
This group is the only English subbing group working on what is probably one of the most touching romantic shoujo series of all time.
ARR - http://arr-soarin.blogspot.com
Older series that they are working on: (They're working on a lot of series... there's too many to count...)
I know I'll probably get a lot of flak by listing them because depending on who you are, you either hate them or like them. I personally like what they're doing, even though their methods may be a bit unorthodox at times.
Kiteseekers - http://kiteseekers.blogspot.com
Older series that they are working on: Mizuiro Jidai
I am aware that Kiteseekers mostly works on newer anime. However, their work on Mizuiro Jidai deserves mention and is one of the series that deserves subs, but got stalled.
ray=out - http://anime.rayout.org
Older series that they are working on: Maicchingu Machiko Sensei, Yoiko
Again, this group is known for their work on newer series, but the two older series of mention are Maicchingu Machiko Sensei and Yoiko. I have not watched either one (as I tend to avoid ecchi), but the overall work is very good.
CandyFukkatsu - http://daniellippert.ch/candycandy/candycandy.html
Older series that they are working on: Candy Candy
This is probably one of the most ambitious projects out there. To sub Candy Candy with lots of audio and subtitle tracks is quite a feat in itself, and they certainly are determined to do it!
C1Anime - http://www.c1anime.net
Older series that they are working on: Perrine, Peter Pan no Bouken, Daddy Long Legs
Most people know them for their work on Peter Pan no Bouken and Daddy Long Legs. Their work is solid and a shining example for everyone else to follow.
Honobono - http://www.honobono.cc/
Older series that they're working on: Goldfish Warning
I can't believe I forgot about them, considering that Tokyo Mew Mew was one of my very favorite series.
I've probably missed out on quite a few groups myself.... so if I did, I apologize in advance...