Drupal's default registration process is more complicated than it should be. To simplify the process I like my users to sign up and log in with only their email address and password.
There are various ways to deal with that issue, with Login Toboggan being one of the most popular ones. However, I rarely need all the other stuff that comes with this module and I prefer small modules that focus on a single task. Besides, Login Toboggan only allows to login with an email address as an alternative to the username, it doesn't remove the need to create a username in the first place.
So what I found best for my needs is a combination of the Email Registration module and a tiny custom module.