On the Special:Userlogin page, "E-mail new password" requires people to remember the username first. It would be a nice enhancement if there was an "E-mail username" link, taking you to a different screen to enter your email address, which would then send a list of the username(s) connected with that email address. Is that possible? Might cut down on the number of unused accounts that get created by forgetful people.
There is a privacy issue I can think of: the page after you click the final button "Retrieve username" should display the same message whether there was actually any name associated with that e-mail or not. Something like: "Thank you. If you registered an account using this e-mail address, it will be sent to you." Otherwise it would be possible to find out if someone had registered if you knew the email address they commonly used.
I guess it would involve a couple of boring bits of programming (new HTML form, localization), but also a fun bit (the database query to get the usernames). (Or is it just me who thinks that? Always liked databases. ^_^)