Inkscape is an open source vector-graphics editor, and thus, it makes a nice counterpart to The GIMP. The GIMP is great for photoediting, but to do nice, crisp logos, you really need Inkscape. Think Corel Draw or Adobe Freehand, only it's free.
From the Inkscape web site:
Inkscape is an Open Source vector graphics editor, with capabilities similar to Illustrator, Freehand, CorelDraw, or Xara X using the W3C standard Scalable Vector Graphics (SVG) file format. Supported SVG features include shapes, paths, text, markers, clones, alpha blending, transforms, gradients, patterns, and grouping. Inkscape also supports Creative Commons meta-data, node editing, layers, complex path operations, bitmap tracing, text-on-path, flowed text, direct XML editing, and more. It imports formats such as JPEG, PNG, TIFF, and others and exports PNG as well as multiple vector-based formats.
Unfortunately, Inkscape isn't part of the basic Ubuntu install. (I think it should be, though.) To remedy this situation, just do this:
sudo apt-get install inkscape
Sample Inkscape screenshot:
And, no, I didn't make that drawing. Beyond my capabilities.