Never reveal your passwords to anyone, ever.
Your username and password are personal information and in a corporate environment you are personally accountable for the account. If someone gets a hold of your password and uses your account for malicious acivities, you are held responsible!
There is a common cracker technique called “social engineering” that can be applied to try tricking you into revealing your password. It could be a person calling you posing as a help desk person, stating that he wants to install some software or fix a non-existant problem on your computer.
Social engineering also often includes mail or advertised programs that requests that you enter your username and password.