Now let me unpack that statement a bit: I am obviously not saying teachers physically or literally take things from others without permission. What I mean is: if I see a great lesson in front of me, or I hear about a great method, or I read about a lesson plan that went well, I am probably going to use it in the future. In point of fact, some of my best lessons come out of reading the blog of Dan Meyer, a PhD student out in California who used to teach math and who wants to improve the profession for all.
That being said, reading about lesson plans online or in books is only one way to gain this knowledge. One of the best things you can do is quite simple: watch someone else teach. It could be in your school building or elsewhere; in your subject area or not. But make sure you spend time doing so in order to see what other people are doing.
For teachers within the School District of Philadelphia, there is actually a policy that was created in the 1970s to help encourage you to do visit other schools across the city (and country). If you look at Board Policy #308, section 2.8-2.10 you will see how we have been given permission to take two observation days a year to see what else is out there.
I highly encourage this practice. Two years ago I took a day to visit a colleague at Science Leadership Academy because we were both using a computer program to augment our math instruction and came back with great ideas to implement. It was wonderful and easy!
As a brief warning: this year the coding system for inputting absence days has changed and secretaries might not think you can take these days off with pay - that is false. The code for something called the TPERS is "17" and still exists. If you have any issues, please contact me or the group Teachers Lead Philly - they have intimate knowledge of this as well.