Chances are that if you have a website, you want users to participate. They either create your content (forums, Digg, Reddit, ...) or help promote your content (blogs, review sites, ...). Users are important for a successful website and you need to make them be able to contribute as easily as possible. Here are a few tips about handling users.
Lower the registration barrier
Make it as easy as possible for a user to contribute. If possible, do not require the user to register: people are tired of registering for yet another website, they have plenty of accounts and passwords to manage already. Blogs do it, StackOverflow does it and many others also understand the value of not requiring users to register. Sometimes it's not possible to allow anonymous participation, in those cases you should consider allowing users to use an OpenID. Now that Google and Yahoo act as OpenID providers, more people than ever have an OpenID account (even though most of them don't know it). Let them know that they can log in with these accounts and you might get a bunch of new users willing to participate.
If you absolutely need users to register, make this step as simple as possible. Don't ask for too much information and don't ask for information you don't absolutely need. The absolute minimum information you need is a user name, an email address and a password. Once you have that, you can probably create a valid account (sometimes you need more, but ask yourself before asking for more). You can have a second step to gather more information about the user, but consider making this step skippable while encouraging the user to fill-in the information later.
Handling registered users
Let them change their user name
One thing that annoys me a lot on websites is the inability to change the user name once the account is created. Sure there are some issues with that freedom, but ask yourself what those issues are. In most cases there are no issues, in those cases you should let the user change its user name as often as it want as long as it's unique. In the case of forums or websites where the users participate a lot in discussion, it can be problematic to let a user change at will. Some forums let the user change their name, but not too often (once every X month can work). For a short time after the modification, the forum also specifies that the user used to be known as "Previous User Name". That way a user can't change its user name to hide.
Make it easy to log-in
This is a given and usually not a problem. Always offer a "Remember Me" option and make sure the tab order is properly ordered so that users don't need their mouse in the process. The usual tab order should be as follow (slight variations can work too): user name -> password -> remember me check box -> submit button. If the focus goes on a "Forgot my password" link (or any other link) between one of these four elements, there's a problem.
It shouldn't be difficult for users to be a part of your community. If registering or logging in is difficult, you will lose a lot of potential users. Make it easy for them to contribute and both of you will be happy.