I have a solutions for this problem, using xml (ServerHTTP).
Single-signon system demo:
http://aspspider.ws/ycicom/xmllogin/login.asp
user:admin
pass:uban694t
user database is stored on
http://thuyquynh.somee.com/loginsys/
Storing file in another server :
http://thuyquynh.somee.com/?list=str
user:admin
pass:diemyeudau
You can upload files to another freehost storage (1asphost.com/brinkster.com/ipdz.com/...), and these file-links is managed on mainsite database.
For Storing file in another server solutions, I use a multipart-form(include file field,text field...) in order to "post" to file-storage server. File-storage will save file(and needed info) on its db, then it will "post" back other infos to main server.
Dont get strange because of my english, hix, Im only a vietnamese student,so my english is very poor.
About multi-domain single-sign on system, when user login on a sub-site, I use xml to post username&password to login server, then login server will return true(follow by user'info) or false. The rest will be solved by sub-site.
I hope you would understand, hic hic, maybe I will get a english course later
You can use any dababase type, even ms access, it would be a low-cost solutions( for a student like me

).
If you'r interested in my works and want to get multi-domain single-signon asp source, please send me an email (ycicom@gmail.com)