A commonplace book is a personal collection of quotations, anecdotes, observations, links to articles, poems, etc. that I come across during my reading and want to save for easy access later.
I've kept a commonplace book for a long time, but I was inspired to put it online by Wes Hill's excellent tumblr commonplace book, and Brett McCracken's linklog-style commonplace book. Both have moved away from this form as they've had more of their own writings to share. But it gave me an appreciation for them by exposing me to the breadth of their influences.
For information on keeping a commonplace book, check out this article.