As stated elsewhere, I am not a professional graphic designer and do not claim to be one! That said I have often found myself in the position of the 'most plausibly able to pretend to be a designer' role in various projects. This site is one example, the logo for Taverna 2 shown below is another:
The logo was created from scratch in Adobe Illustrator CS3, along with various other graphical assets used by the Taverna project.