Building a custom PC comes to mind (although this has changed a bit) as does buying on furniture and refinishing.
I also note that if your money saving via DIY enables you to increase your household savings rate, the compounding nature of the investments can bring in a huge amount of money in retirement.
But even a mediocre cook with a limited menu can eat better food at 1/10th the cost by cooking it themselves.
Not only can you save insane amounts of money, you gain a confidence in the result that you cannot get any other way. You know what's in them walls, it's no big deal to go back into them when you need to.
Often further savings are available by using salvage materiel, as you have the time and involvement necessary to separate the scrap from what can be used. You can't really skip this step with "new" anymore, anyway.
Second on my list is pretty much anything related to automation or IoT.