I am looking for an open-source project, written in C#/Java, that does vectorial graphical manipulation, a-la Illustrator. I don't need anything terribly complex or fully-featured. I intend to use it as a base for creating a 2D-game level editor, where I will be manipulating paths to create some geometry for the level.
I figured that what I want to built is extremely similar to a painting program using vector-based graphics. I welcome any suggestions.
I believe you mean ala Illustrator as Illustrator is vector based, whereas Photoshop is raster based. I have used Illustrator for this kind of thing in the past, works great, but obviously is not free. I am not sure of a good free vector graphics program unfortunately.
After a brief search, I see a couple of likely candidates. Inkscape (http://www.inkscape.org) looks promising for what you are looking for. As well, possibly too simplistic, but GoogleDocs now has Vector based documents available.
Neither MS-Paint or Photoshop allow vector graphic manipulation, however Inkscape allows for vector graphics manipulation of SVG files. And it's free, easy to use, and well documented. If you want something more Photoshop like (i.e. it allows you to manipulate bitmaps), then try Gimp - also free, well documented, and relatively easy to use.
I took a course in Game Design last Spring and we created our level editors using XNA game studio. Windows Forms actually makes it pretty easy to develop the graphical interface.
Here is a list of downloadable demos that you can use to help you out. The level editor here sounds kind of like the thing you're looking for:
http://www.cis.cornell.edu/courses/CIS3000/2010sp/resources/demos.php
ImageMagick should do exactly what you want.
http://www.imagemagick.org/script/index.php
They provide the source and binaries.
Here is the link to the source:
http://www.imagemagick.org/script/api.php?ImageMagick=vbs078aa3mje21nulk7hd0eb93#dot-net
There is a version for .NET in the list as well as java.
I hope that helps. Cheers