I am using EAGLE to draw schematic diagrams of the CPU. The reason I chose EAGLE was simple – everyone uses it, it is free (or inexpensive under non-profit license), and has a rich component library. Also, most PCB manufacturers accept EAGLE files, and if not, there are many scripts to output gerber files. So, if I ever decide I am ready for PCB, it should be fairly easy to draw one, too.
Unfortunately, EAGLE is not so easy to start playing with. The GUI has some bizarre features you have to be aware of. Normaly for a beginner it takes a while until you are able to add a component to your schematic and know what you’re doing. When I was looking for some kind of EAGLE tutorial I came across this on YouTube:
http://www.youtube.com/watch?v=qG0O9LKH-_E (lesson 1)
http://www.youtube.com/watch?v=z4Io4qZCzYI (lesson 2)
http://www.youtube.com/watch?v=G0bMfV3IbMo (lesson 3)
http://www.youtube.com/watch?v=rGiHdNyHbDM (lesson 4)
http://www.youtube.com/watch?v=3Xj1Xrp8uso (lesson 5)
http://www.youtube.com/watch?v=UW_Jkzuo_mc (lesson 6)
http://www.youtube.com/watch?v=TmV9urSuSFU (lesson 7)
http://www.youtube.com/watch?v=4JC32tq7Ibo (lesson 8)
http://www.youtube.com/watch?v=4xjjahxMapc (lesson 9)
http://www.youtube.com/watch?v=cB5Z_fRpLrE (lesson 10)
http://www.youtube.com/watch?v=tna0XNjHkrw (lesson 11)
It is a series of 12 lessons (although lesson 12 still to be released as of today) led by a guy (Jason) who does it for free to give people like ma a smooth start with the software. Lessons guide you through EAGLE following a design of a simple circuit, from schematic to board. Each lesson is less than 10 minutes but I have found them amazingly comprehensive, and very useful. I watched them all and was able to get started seriously with EAGLE immediately afterwards. Thanks Jason with rpcelectronics!