Are there any alternatives\mods to .net Membership?
I find it quite restrictive;
Cant change Username, easily. You have to create a new user and copy the fields, but then you lose the primary key OR you have to edit the user table directly yourself.
Additional profile fields are stored together as one blob.
As for the Profile there are a couple of alternatives out there. These two use either a table or let you call a stored procedure. Of course you can also implement your own. I personally got tired of using the Profile Providers, and found that dealing with the profile in my code was easier to control and contain.
As for the other issues, you can also implement your own provider. Microsoft released the source code to the SQL Providers so it can give you a starting point.