What are DIY projects that can use technical ability and/or time to obtain something that typically costs much more as a paid product?<p>Building a custom PC comes to mind (although this has changed a bit) as does buying on furniture and refinishing.
Does home cooking count as a DIY project? The ROI is huge.<p>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.
Home building.<p>Not only can you save <i>insane</i> 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.<p>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.