SMD package size chart
In my experience, a printer is the most useful design aid / sanity check. For example, I use Eagle to design my PCBs, and there are reference packages available for a very large range of devices. Simply printing out the design 1:1 helps a lot. It can also help you see things like "Those parts look like they would be hard to solder/probe" and "These parts are too close/far away". It's also cheap and fast. Some manufacturers will put 1:1 footprint drawings in datasheets as well, and you can use those for reference.

As for a specific design aid, I also have the SparkFun Reference Board, which helps you see what holes, text, and common packages look like on a footprint. I find it useful at times, especially when I'm trying to figure out readability of text.