Both the miniExtensions Form and Portal extensions give you the option to have your users pick login details from a list, instead of entering data manually. This can be useful when privacy isn't a main concern, or when using the login more like a record lookup (e.g. look up a product record from a catalog).
Here's how to set this up:
For Forms, ensure that the Login page option is selected under How do users access their records? in the Share section. Portals will always have the login page enabled.
In the Share (for Form) or Users Table & Login (for Portals) section, click on your login field.
Turn on Allow users to search from existing values in the table.
Your users will now see a magnifying glass icon above that login field that opens a list of possible values. This list can be filtered as described here.