Jan 2017 Update Sadly, Nitrous.io is no more. There is no longer a company and infrastructure behind it. In 2016 one of the founders mentioned a possible open source release of their platform.
Minimal usage is free. More serious usage starts at around $20/month.
There is a point system. If you do certain things, you get more points. Points are not expended, but are used when a virtual dev server is created and released when it is terminated. Some points are free, and you can get to enough points to have a free box, but the free boxes are reset periodically. This is obviously a free trial gimmick, but not a bad one. For $20/month you can have ~4-5 development boxes if you are reasonable on resources. Unbooted boxes count against resources. Deleting or Initializing boxes takes less than 30 seconds, and the choice of locations includes US East, US West, Europe, 2 in Asia, South America and Australia.
I have been using Nitrous.io for developing a client's project in MeteorJS now for a few weeks. It allows me to develop and test from any computer in any room of the house.
How it meets your requirements.
Linux compatible (Gnome based, essentially Ubuntu)
Although it does not require Gnome, it is web based and does not discriminate against Linux.
Netbook Friendly (Easy on resources, small screen)
Nitrous.io is web based, you need a web browser and internet connection to use it.
It will not work offline, as your code is being edited and executed on Nitrous's servers.
Multi-Language support as I will likely be using HTML5, JS, jQuery, Perl, PHP, > Python etc. The main goal here is to learn.
It supports container-based web app development in PHP, Python/Django, NodeJS (including Mongo), Ruby, and Go. Obviously you can also edit HTML, JS, etc.
Most environments have common languages like Python.
Focus is on web development
Yes. You can preview your site by running it in the console and clicking a preview menu to be taken to an SSL secured tunnel to your site.
Features I would like:
Auto-Indent and code assist (Again, it's about learning)
Auto-Indent exists. It works well for Python.
Code assist, like go-to-definition seemed better on Cloud9's hosted editor c9.io
You can link Cloud 9 to Nitrous if you are willing to pay for both....
Easy to use but not WYSIWYG
Very easy. You can add about 50 different packages to your dev environment from a pull down menu. If something you need is not on there though, you don't have root access, and need to install it in user mode. So far that hasn't been a problem. For example, to install Beautiful Soup in python within a user account you can
pip install --user bs4
With Nitrous.io, you get a decent web based IDE. It can go full screen either for the console or the editor. The editor has language-specific behavior.
You can also add a key to
~/.ssh/authorized_keys, and login with ssh from your netbook. You can run commands, or say, emacs that way. You could use scp to move files in and out, or use git, curl or wget. There is also a way to deposit files via the web.
Built in documentation and/or community support
Docs are OK. Design is natural and it was obvious what everything did, so I did not refer to the docs much.
Has forums. Good, fast, friendly support team, too.
One Friday evening my Nitrous.io virtual server was stuck in "booting-not ready" but support had it fixed in well under an hour. For a Friday, I was impressed.