Processing is an art computing environment that has the (dis)advantage of being based on Java and capable of running in any web browser with a non-free Java plugin (which really needs fixing, guys). There are better languages and environments for art computing, but there are none with greater mindshare or more active communities.
There are a couple of websites that allow you to create, upload and share Processing "sketches" (programs), which is great. Processing was designed in part as a successor to the "Design By Numbers" educational graphics programming environment, and its use in design and generative art makes sharing code good for the progress and development of culture.
sketchPatch is very easy to use and allows you to write, compile and run Processing sketches in the browser. Sketches are licenced under Creative Commons’s Attribution-only licence. Which CC don’t recomment for code. MIT or the revised BSD licence would be better for a permissive/gift/psuedo-public-domain licence, and the GPL would be better for ensuring that people can use the code they contribute and build on.
OpenProcessing allows you to upload and share your sketches. The licencing is better than sketchPad – code is GPL the graphical output of the code presented on the website is BY-SA. But they do need to make it clear that the code is GPLv3 or later, and that BY-SA covers the non-code assets rather than the code, and that output from sketches is only BY-SA if presented on the site or it incorporates pre-existign BY-SA assets.
Neither site is Franklin Street free, you can’t get the source code for the websites/web services themselves.
Both sites are great resources for digital artists and designers. Tuning up the presentation of their licences, and ensuring they are free network services, would future proof them for their creators and their audiences.