This is an old question, but sharing what I found recently (Nov 2021).
First of all, there are no CPU Design only options. There are general technical drawing options and most of them are usable, but with some difficulties i.e. without the exact shapes for adders, no good connectors etc.
But with some patience I think you can use them all but the 2 free ones I think are the best are - Inkscape and Libreoffice Draw. Visio is a good one if you can afford it.
Inkscape is very nice to draw shapes and use generally. Only problem is with connectors and there is some work going on to make custom placeable connectors to make this work better. This thread has more details and this page is the issue page.
But if you scroll down in the comments, someone mentioned that Libreoffice Draw works well and they've described how to use it with an example as well:
LibreOffice Draw has gone some way towards implementing the features described above. However, they do not support connecting to the connection points of children within a group, meaning you must un-group, make a connection, then re-group in order to edit diagrams like the one below (each named port is a text-box with a dedicated connection point, which are all grouped together with the block symbol). While LibreOffice appears to be a step or two ahead of Inkscape with respect to connection points, I find Inkscape to be far superior in many other ways. If Inkscape could implement this feature, it would become my preferred drawing tool for all diagrams.
I gave Libreoffice Draw a try, but I prefer Inkscape interface more as I'm used to it.
Here's a first attempt at making a schematic, I had to create the shapes and the connections using Bezier tool with the
Ctrl key pressed. It takes a bit of time, but I think it might be better than drawing on a paper (which is what I've been using!)