What are my options for rendering graphics encoded by Java2D to vector graphics formats such as EPS, SWF, SVG, VML, PDF and others I might not have heard of yet with open source libraries?
Batik is the most widely-used library for rendering SVG. What other options are there?
iText is widely used for rendering PDF. Is there something less functional but possibly simpler?
For rendering SWF (Flash) from Java2D, I've found Transform SWF from Flagstone Software very effective, if not intuitive.
Which less-popular libraries, or projects starting out, would be worth looking at?
Batik and iText are both good libraries. I've also tried Apache PDFBox, but I don't think it has support for Java2D.
Regarding iText, although it has extensive capabilities, you can still use it for simple Java2D rendering tasks by way of the PdfGraphics2D class. For a short example of how straightforward this is, see this link.
Here are some other resources that you might want to explore: