As Wikipedia says, commonplace books (or commonplaces) were a way to compile knowledge, usually by writing information into books. Such books were essentially scrapbooks filled with items of every kind; medical recipes, quotes, letters, drawings, poems, proverbs and prayers. Each commonplace book was unique to its creator’s particular interests.
Now much of our exploration of the world is achieved through our browsers and things have changed hugely since Francis Bacon wrote the following (one of my favourite pieces of writing).
TRAVEL, in the younger sort, is a part of education, in the elder, a part of experience. He that travelleth into a country, before he hath some entrance into the language, goeth to school, and not to travel. That young men travel under some tutor, or grave servant, I allow well; so that he be such a one that hath the language, and hath been in the country before; whereby he may be able to tell them what things are worthy to be seen, in the country where they go; what acquaintances they are to seek; what exercises, or discipline, the place yieldeth. … Let diaries, therefore, be brought in use.
For those of us who use the Internet and rely on it for information, sites and services such as Twitter, Flickr, Tumblr, Instapaper](http://www.instapaper.com/) and Readability and applications such as DEVONthink are our commonplace books.