If the user's name is "John Smith", here's what the different options would look like:
first.last - john.smith
firlas - johsmi
The 2nd is what we use on this site because it doesn't directly show a user's 'real' name and creates unique usernames for people. It works well for most users, but it depends on your site.
If you have suggestions for other automatic generations of usernames you'd like to see, just let us know
Thanks,
Alex