PLNs allow you to work on your professional development, find resources, learn new ideas, network, keep up-to-date about the lastest news in your field, among other things. TEFL teachers have really taken to PLN.
Blog Carnival: You can just read it or write your own post and have it be included in the carnival. Here's the ELT Blog Carnival and upcoming topics for it.
Blogs: Start following a couple of blogs and commenting on them. You'll get to know the blog writers and other readers that way. If you don't have a blog, consider starting one. Edu Blogs is probably the largest blogging community of educators out there.
Facebook: Join groups or networks and actively participate in them.
Linkedin: Complete your profile and start connecting with people. You might want to consider becoming a LION (LinkedIn Open Networker) and accepting any and all invitations that aren't spam. Joining networks and participating in conversations can help.
Nings: EFL Classroom 2.0 and the Educator's PLN are good places to start.
Podcasts: The itunes store has good podcasts, some of them are even free.
RSS: Allows you to follow blogs easily.
Social Bookmarking: You can see what other people are reading online and also share your bookmark list with others.
Skype: You can talk to people around the world.
TED: Lots of great presentations by famous people.
Twitter: Short and sweet, you can share ideas anytime, anywhere.
Web 2.0: This is basically a combination of many of the ideas here, social networking, blogs, wikis, videos, etc, all in one place.
Webinars: Some publishing companies have free webinars that you can attend as long as you sign up.
Wikis: More and more groups are creating wikis in order to gather information in one place.
Youtube: You can subscribe to channels, watch the latest presentations, and comment on them.