Ok I don't have any really frugal projects like that!
My frugal home improvement projects so far amount to buying items like a ceiling fan at a garage sale. ($5
)
BUT... I have seen MANY things for sale at garage sales that people bought too many of when they did some remodeling. This includes light switch plates, new paint, wallpaper,insulation, wall board, sinks (NEW!), faucets, vinyl flooring (enough for a hallway or small bathroom), light fixtures( many people replace perfectly good ones when moving into a new home), doors, and tons of nails, screws, and other hardware. I bought new drawer pulls for my daughter's dresser and many hooks good for closets or in the bathroom. I also found brand new sponges for sponge painting, paint rollers, a "wall art" kit, brackets and standards for shelves, and tons of baskets and organizing containers.
Church rummage sales have lots of remodeling supplies (usually used) really cheap.
If you are thinking of a big project, look in the classifieds. I have seen things cheap and sometimes free. Sometimes they want you to take it apart or just want it hauled away. These might include wood, fences, dirt or other landscaping items, and fixtures like tubs, sinks, etc.
Also, check with your city's recycling program. Ours offers supplies such as paint for free. These are things people drop off and are still usable. Of course, your choices and the quantities may or may not work for you.