If I am writing internal applications with a modest number of users then I usually rely on asp.net memberhsip. Its familiar, I know how to get it up and running quickly. Other developers also know it reasonably well so it doesn't need explaining. Its also usually desirable for the organisation to be in control of user accounts.
On the other hand, if you were creating a public site and needed to authenticate users before making contributions (eg. stack overflow), you might want to think about implementing OpenId so users don't have to create a new account with your domain. They could use Google/Facebook/Twitter instead. There are libraries to get you started.