Onshape.

Its new, its amazing, and its powerful. You can do basically everything 123d design can do (my second choice for easy software, followed by google sketchup) plus it works like google docs, where more than one person can work on the file at once, live.

Best part, its free, and its alllll browser based. so you can access it on ANY platform, even your phone. and make changes, and coordinate with other artist all live in the document.

Also you can mate objects and assemble them in software to check for fit and function etc. its a must check out.

Google sketchup however is also a great program I highly suggest that everyone has a copy of that on their computer. Really good for dimentional layouts.