OER repositories contain more than just open textbooks - many of them also contain Lesson Plans and resources that can help with writing a lesson plan. With rare exception all content linked to from here is free but usage rights vary. Most materials in these repositories are released under various Creative Commons licenses, while some are in the public domain. Many of these sites are collections of collections, some have been updated recently and some have not.
AMSER (Applied Math and Science Education Repository)
Portal of educational resources and services primarily focused on math and science education. The link goes to their Search page, use ‘Lesson Plan’ as a keyword search. (Each of the search boxes is a drop-down menu showing additional search options. Click to see their resource categories.) The results page uses > and >> at the top of the page to advance to the next page of results. Additional options for narrowing that particular search are on the left.
FIND OER start.me
This free start page for finding OER resources includes open images, textbooks, course materials, and video/audio. The third column (Open Course Materials (A-Z)) links to a number of OER and OER-like repositories which may contain lesson plans, several of which are represented elsewhere in this list.
Internet Archive
The Open Educational Resources section of the Internet Archive seems to be sparse in lesson plans, but searching 'lesson plan' in the search box in the upper right produces some potentially interesting results.
Lumen Learning
Click 'Course Catalog', then select the general field in the 'yellow to green' bar at the top. Click on a potentially relevant course, some of the courses will have a 'View Course Content and Outcomes' link in the upper right, which provides a page of links to each section of each module for that course - essentially a collection of lesson plans for that course, some of which will be useful to you.
MERLOT II
Free, peer reviewed collection of online teaching and learning materials maintained by the California State University System. Use 'Lesson Plan' and additional search terms and then narrow the search using the options on the left or the tabs across the top. There is a 'sort by' box on the right of the search results.
MIT OpenCourseWare
A web-based publication of materials from thousands of MIT courses. OCW is open and available to the world and is a permanent MIT activity. Search for your topic, then narrow the search results using the options on the left and switching between 'Courses' and 'Resources' to find ideas or information for writing a lesson plan.
OER Commons
OER Commons is a library of OER materials containing over 50,000 high-quality OER. To find lesson plans or similar material try their default search, specifying 'lesson plans', your field, educational level, and - if applicable - education standard. Also try their Advanced Search - click any/all applicable boxes and click Search in the lower right. With either default or Advanced Search you can filter the results using the options on the left.
Openstax CNX (formerly Connexions)
Contains textbooks that can be read online or downloaded for free. The textbooks come "with free resources for educators (like LMS course shells, lecture slides, instructor answer keys, and more)".