Cheap Thief Mission Downloads is the premiere collection of Thief fan missions on the 'net; while it may not sport the prettiest interface, it provides the largest repository of Thief-related material available.
But enough hype. If you're a mission author, send your mission to me and I'll host it.
If you're developing a mission, send it to me and I will host it in a secret location outside of the main archive that you can give out to your testers. Updates are no problem; just send them to me and I'll put them up. In addition, I can create a private discussion area in the Cheap Forum. Your testers will need to register and you will need to send me their user names so that I can give them permission to view and post to your forum.
There are basically six ways to send me a mission:
My email address is firstname.lastname@example.org.
The FTP upload site is upload.thiefmissions.com; use the user name "ftp" and any password you wish. Place files in /pub/incoming/thief. Nobody can download from this directory, so your files are safe from people snooping for betas. After uploading files, send me an email and let me know what I should do with them.