If it's just undervalued tech, and not "what's the most powerful tech that's obscure" I'd say it's the PNG file format. Lossless, greater color-depth, and alpha-blending: PNG would make the most difference on the most number of average people's web pages if they used it. Some of this other stuff is a lot more powerful, but it doesn't get my vote for undervalued because I think the audience is anybody who is creating a web page, not just tech guys.