You can now use the OAuth protocol for authenticating Web applications that access data in a Salesforce.com instance. OAuth is an open protocol that allows secure authentication and is often described as the valet key of software access. A valet key only allows access to certain features of your car: you cannot open the trunk or glove compartment using a valet key. Similarly, OAuth limits access to a software application, without having to hand out the user's username and password.

For a nice introduction to OAuth, see this page.