Is there a NodeJS plugin for Aptana Studio?
We have no NodeJS support currently. If this is something the community is interested in, it'd be helpful to file a feature request and vote it up: http://jira.appcelerator.org/secure/CreateIssue!default.jspa
Since we are built on eclipse, you should be able to try out the instructions for NodeJS debugging on Eclipse, found here: https://github.com/joyent/node/wiki/Using-Eclipse-as-Node-Applications-Debugger
As for special NodeJs projects, there's no notion of that. You would likely just create a normal Web project. We do have the concept of libraries that you can add to a project, wherein you could point to js files/libs for NodeJS. We also has special syntax files for describing JS libraries/APIs so it can be integrated into our content assist. A good example might be the ruble we have for jQuery which contains that file for two versions of the jQuery API. Here's the 1.6.2 version: https://github.com/aptana/javascript-jquery.ruble/blob/master/support/jquery.1.6.2.sdocml The bundle.rb up in the parent directory hooks up the file in the ruble/bundle.
My guess is that creating an analogous NodeJS ruble and building up an sdocml (xml) file that described the API would be the easiest way to get started. Sharing that on github and sending it to us would allow for others to contribute as well. There are docs for creating rubles here: http://wiki.appcelerator.org/display/tis/Creating+a+new+Ruble