It seems like i am advertising for inkscape but this is really powerful.You can design logos,banner,web templates very easily.For downloading you can visit here
Inkscape download
You can find many tutorials from this site and youtube.Deviantart contains a lot of tutorials on Inkscape too.Inkscape has many rich features to work with nodes and paths.
Here are two sample works that I made using Inkscape
Plumber
plumbing
I am using this tool for only 1 month.Tryin to learn more inkscape. Try Inkscape have fun.