Both http://sharelatex.com and http://overleaf.com support that by re-rendering a pdf preview as you make changes. Overleaf also has a WYSIWYG editor with inline latex, although I find it doesn't work for everything.
https://cloud.sagemath.com/ has one of the best online latex editors I've seen. It has live preview and you can jump from source to the right position in the end product, and vis-versa. It's definitely worth checking out!
Texmaker is pretty close. It doesn't update live, but I've never actually needed that capability. Tons of other useful features too. No affiliation, I just use it a lot.