A great number of web sites offer access to FREE! lesson plans in every subject area. Unfortunately, the quality of these lesson plans vary as much as the web sites themselves vary. EduHound is one web site that has tried to search through the internet's pile of lessons and provide links to the better one. Unfotunately, EduHound provides no insight into its criteria for selecting lessons.
Nevertheless, EduHound does provide a list of mathematics websites at both the middle school level and the high school level. It is up to the discriminating teacher to look through the suggested lessons (approximately 150 for the two levels), select those that have promise, and then adapt them to his/her own style and use.
For example, at the high school level, the lesson A Look Back seems to have promise. Its objective is to "recognize the work and contributions of women to the field of mathematics," by providing an interesting lesson on Sonya Kovalevsky, a less-interesting (in my opinion) on Hypatia, and an interesting lesson on "Navajo women’s abilities to conceptualize mathematical constructs in their unique weaving." The series of three lessons (written by Laurie Beauvais) include the pertinent mathematical developments, assessment questions, and copies of hand-outs.
Useable at both the middle and high school levels, another lesson plan with promise is Sharla Shults' Does the Decimal Point Really Make a Difference?. The lesson discusses the problem with advertizing cheeseburgers for 0.99 cents each and then moves on to a study of the "decimal dilemma" responsible for major mathematics mistakes in the real world.
Specific to the middle school level (though certainly useable with high school students", the lesson Magic of Mathematics: Patterns is suggested. The lesson asks students to examine magic tricks done using a deck of cards or dice, in order to determine the underlying mathematical patterns.
A side benefit of EduHound's work is that it sends you to actual lesson plan sites, which then open the door to other sites, etc. That is, once you find a gem, other gems perhaps lie in the same vicinity.