This is a short list of workshop-type environments I've found, mostly by doing keyword searches at Yahoo! Groups. If you have one that belongs here, let me know, because this list is by no means comprehensive. Not all of these lists are strictly workshops, but they all encourage constructive criticism.

Buffy the Vampire Slayer Writers Guild
This site acts as a resource for slash and het writers. It includes a list of beta readers, character and genre "zones" and a host of other resources. Great idea.
Members: ?

SWFF
Description:
Wish to grow as a writer of Star Wars fan fiction?? Here is a safe place, where all authors can feel free to share their original work with others, in hopes of honest comments, critiques, and idea exchanges about their work. This list is for Star Wars Fiction Fans of all kinds. Gen fic to AU's to Smut to Slash; all are welcome! If you wish to share your stories with others or you just like to read them, head our way, but only if you heed this warning. Intended for a MATURE AUDIENCE ONLY, 18 or older. Come share tips, story ideas, writing frustrations, advice. This is a list for those who love to write. Constructive criticism friendly, we are open to all Star Wars fiction, though are presently Episode one obsessed. We look forward to your contributions!
Members (as of May /01): 124

Racine Street
Description:
This group is for people who enjoy creating fan fiction related to the television series Due South. We read and post fiction, give and receive constructive criticism, and enjoy chatting about the show. We are primarily writers of gen fic, though writers of slash fic are most welcome. Personal flames will not be tolerated. Constructive criticism is always welcome. Over 18 only, please.
Members (as of May /01): 43

BtVS_Angel_Fanfic_Beta
Description:
This group is for two kinds of people writers of BtVS or Angel fanfiction who want constructive criticism on their fics in order to improve; and people who LOVE reading that fanfiction and are willing to HELP these writers improve. To join, you must, at the very least, fit into the second category. The main function of this list is to provide a place where Buffy or Angel writers can get their fics beta read. So, do not join this group if you are not willing to critique people's works. If you're a lurker, this is not the group for you. If you join this list, you MUST at the very least provide constructive criticism on some of the fics posted here.
Members (as of May /01): 9

Lovofmeandthee
Description:
If you love Starsky and Hutch together, as partners, friends and/or lovers, this place is for you. If you DO NOT like reading, writing, or discussing slash (m/m) stories or issues pertaining to such, please DO NOT join. Hopefully, this will be a fun and educational list for all. One other thing - if you are not interested in improving as a writer, please don't join. Honest critiques of ALL stories are allowed and encouraged. No flames, no personal trashing allowed.
Members (as of May /01): 168

Dreams_N_Print
Description:
Tired of 'boyband' fiction groups that are a clearing house for bad writing? If so welcome. Here, our focus will be improving our writing with real feedback and challenging topics. Writing samples MUST be submitted to join. Participation is somewhat expected. While we realize people have lives outside of the fiction world, we are all writers and enjoy reading others' work as much as reading comments on our own. We expect equal participation in the way of writing submissions and feedback. Slash, het, gen... we want to see it all. The only limit is that the fiction must be about one (or more) of the current groups. However, if you have another topic of fiction you'd like for us to read over and critique, please ask us -- we will more than likely say yes. ALL fiction must be labeled as far as genre and group focus. Warnings and/or templates are greatly appreciated. If you are looking to improve your writing, looking for some of the best fiction on the web, or just looking for some real feedback on your work, this is the place for you. No archived fics, please. We're a workshop -- we want first dibs.
Members (as of May): 29

Slash Writers Workshop
Description:
This list is dedicated to helping slash-writers become better writers. It provides a place for slash-writers, of any fandom, to present and discuss ideas, plot-lines, fragments, vignettes etc. Stories in alpha or beta condition are acceptable for posting, and will be considered open for comment and debate. This allows the writer to get some honest feedback on a story before posting it to the public lists and newsgroups.
Members (as of May): 85

Fanfiction Critics Association
This is not so much a place to workshop your writing as it is to critically examine different aspects of fan fiction. But I'm including it anyway.

Long Title
Description:
A Monkees fanfiction writers workshop, designed to provide constructive criticism and helpful feedback on any fanfiction stories submitted to the list. Story ratings range from G to R. No smut or slash allowed. Because of the constructive criticism workshop atmosphere of the list, YOU MUST BE 18 OR OLDER TO JOIN. Come join the Monkee romps in the Long Title Library!)
Note:
This list does not allow slash, but I thought I'd include it anyway because it kept popping up on my travels.
Members (as of May): 134