Google's Gmail is a nice little email program, ignoring questions about privacy, of course. For example, there are ways to set it up so you can automatically filter things. If your email address is JuanPena@gmail.com, you can create "disposable email addresses". I could send email to JuanPenaemail@example.com an it would still get to you. You could then give out one version of your email to your friends - the easy one - and use another one for Web sites you trust - and a third version for Web sites that you don't really want to have your email address, but you have to give them one for something.
The cool part is that you can then use Gmail's filtering system to automatically ignore anything sent to the junk version of your email address - so if someone emails JuanPenafirstname.lastname@example.org, you could have that automatically put in to a garbage folder, or you can have it deleted automatically!
Obviously this is not something everyone can do easily. What Google decided to do is create the aforementioned Gmail Ninja site. They've divided things up in to four categories - basically, easy, medium, hard, and really complex - and this way, based on the skill level you select, it will show you tips.
I like this idea because there are plenty of Gmail help sites out there...and Google has taken ownership of it and organized it in a logical way.