As a developer, how does the GPL v3 affect my existing work that I developed under other licenses?

Any code you write and own copyright to, which is not a work based on Aptana Studio or Jaxer code, can be distributed in any manner under any license. However, for example, if you create and distribute a combination of your own code with Aptana Studio or Jaxer, or you otherwise create a work based on an Aptana product, then the whole work must be GPL'd or you must obtain a separate license from Aptana.