FourSquare, or rather location-based social networking in general. Bummer that this category of apps did not work out, had real potential to connect people locally.

I was a big fan of Sparrow (acquired by Google) and DarkSky (acquired by Apple).

I miss them both and think of them often. Sparrow truly changed the way I used email. Once it was shutdown, I went back to the trudging through it.

I was hoping that someone would come along and pick up the technology hinted at by UltraConductors[1]. Image cables 10,000,000 times as conductive as silver at room temperature, made out of plastic!

[1] http://www.superconductors.org/ultra.htm

To start, I really liked the idea behind Muse, but they couldn't find PMF: https://adamwiggins.com/muse-retrospective/

And more recently, buildspace :(

Nothing but respect for all the hackers & founders who tried to put something new out there

I forget their name. You gave them access to your mailbox. They would pick up your mail, digitize it, and give you an email like inbox for your physical mail. It even had a spam filter. It was awesome.

Ubiome was helpful after an antibiotic series helping measure where I was with different bacterias levels and how that changed introducing new foods

Pebble (smartwatch) is probably one of the best stories of company failures despite having amazing product.

I don't know if it was actually a startup but there was once some idea of Wikipedia starting a search engine based on serving the customer rather than serving ads. I thought it was a great idea.

OnLive. They did game streaming and spectating just a few years too early!

Keybase. Crypto primitive digital identity, efficiently delivered at scale.

Be