What is Reddit's opinion of GNU Units?
From 3.5 billion Reddit comments

➔ GNU Units website

By popularity on Reddit, this Service is:

#23 in Calculator apps

1 review of this app found across Reddit:

Good stuff.

If you're looking to add new features, look here for ideas. It's a unix utility which converts units. Source code here in C

obviously a a js version would be easier to add to web projects than one written in C