Email Spoofing is one of the latest online threats which is exploited by a lot of hackers to send anonymous emails from virtually any email address.
Email Spoofing is widely used in Email based Phishing attacks and also Social engineering hacks, though most of them are normally caught in the spam filter of your mail box.(if you are impersonating an email from billgates@microsoft.com etc. you probably wouldn't make it. While sending email from the email id of the victim's friend/colleagues etc. are considered safer.)
There are a lot of online websites which allow you to write an email, and send them from any email id you want to the victim computer, and the victim is most of the time left clueless about the spoof.
Some of the popular email spoofing websites on the net right now are:
though my personal favorite is Emkei.cz as it offers a lot of customizable options like changing the email header, attachments, adding reply address etc. and it is completely free.
~~Spoofing Tips~~
You can send an email to your friend impersonating his teacher or maybe his ex-girlfriend and play an awesome prank, though email spoofing considered Not Legal.