Repl.it is the first massively open computing platform where anyone, even without even needing an account, can hop on and in seconds start executing code, build and host applications and websites, and collaborate with other people.
Providing a service like this is challenging from all a scaling, security, and billing perspectives. Also, supporting the number of languages we do (https://repl.it/languages) is a hard tooling problem -- we have to build generic protocols for executing, debugging, authoring code, running tests, handling files, etc. Finally, it's an incredibly challenging frontend/design problem to build a fast collaborative in-browser IDE that's both powerful and simple.
It's a great time to be joining us, we're still 8 full-time people, we reach millions of coders a month, and we're backed by YC and a16z.