There are several pieces of software available that help you design circuit schematics and circuit board layouts:
Eagle is not totally free. It has a freeware and professional version. The limit on the freeware is the size of board you can build, which probably is not an issue for small projects. Beyond this Eagle is a fully functioning professional circuit design software. It works like many CAD programs. There are Windows, Linux and Mac OS X, versions of the software. The interface can be a bit daunting at first, but if you understand circuit design, it isn’t too hard to go from schematic to board layout.
Fritzing is a very new project, as of this writing (still in alpha as of 1/30/2008) but looks promising. Its back end is actually built off of Eagle and so there for to use it full you need Eagle installed. But it is a bit more user friendly than Eagle allowing users to see designs made of more like real wires and boards than circuit diagrams.