The Linux Foundation
When getting involved with Linux, one of the first things a person should do is to make sure to get in touch with the Linux Foundation. They are the epicenter, heart, and soul of the Linux community. Go to their website and bookmark it into someplace that is fast and easy to get to. If possible, make sure to start an individual account account as well and possibly even consider getting an @linux.com email address; they're very cool.
Next, check out all their great educational programs. They really pack in a ton of areas that would interest anyone and have a continual slew of new topics and information from top Linux people around the world. Classes are available for all levels of skill so there is no worry about finding a course regardless of what level you feel your Linux experience is at. From the greenest of new Linux user to the most experience UNIX admin, there are courses that can certify and teach something new.
Make sure to consider taking their "official" certification programs as having these under one's belt can really help push a career working with Linux along. The need for Linux professionals continues to grow at an exponential rate every year so there is no better time than the present to get on that train and get certified from the people who "know Linux".