There's a nice beginner's tutorial in Natalia's Second Life Diary Blog about how to make clothes with The Gimp! It shows some of the underlying principles in a very convenient Howto manner and the Gimp is open source. So, if you ever wanted to give this a try, you should definitely check it out and give it a try!
Ok, more advanced topics like working with the alpha channel are not mentioned there, but that's ok.