web app - What are some great web-based prototyping tools?

I'm looking for a list of web-based prototyping tools and I'd like to build one up here by using StackExchange's democratic voting system.

There are already some lists around the web, notably these ones:

I found both of those from the Google search "list of prototyping tools".

StackExchange UI should, on the long term, be the best format for managing such a list. Let's go for relevance, not necessarily completeness; completeness would involve adding tons of answers including little-known or outdated tools. Instead, I'd like to see a list of current web-based prototyping apps, and votes for each product would ideally evolve over time (eg. Balsamiq will likely start out at the top today, but 5 years from now we might see something else show up).

I'm making this a community wiki since it's more documentation than objectively answerable.

Answer

HTML + CSS + JS

That's pretty much the only option if you're looking at high fidelity prototyping.

I'm an advocate of going hi-fi (code it) or stick very lo-fi (sketchy apps like Balsamiq).

Unless you're testing relatively tame interactions, I find solutions like Axure and the like a bit dangerous. It implies hi-fidelity, but you can't finesse the interactions in apps like that to the point where it will reflect the actual user experience.