There has been some discussion about the limitations of Copyright and how it can impact the learning materials at your disposal. A new(ish) movement began a number of years ago, called the Creative Commons. Materials under the various Creative Commons licenses are available for use in a variety of ways. Below is a snapshot of the different licensing options:
Below are two lists of Open Educational Resources (materials made available under varous creative commons licenses) that offer lessons and readings to students and other educators. Some of these sites will also allow you to contribute your own content, if you wish to share it with your colleagues.
Keep an eye on licensing and copyright restrictions when using any of the following links, to be sure that you provide proper attribution.
The following OER sites offer a variety of services, but may be more appropriate for high school and college-level instruction.