We often help our customers figure out which metal alloy or plastic will best suit their design. While we are making steady progress on our FAQ's on materials, I thought I would share a few of the great resources we use to help track down data such as density, strength, and thermal stability.
First, we have MakeItFrom.com, a website that lets you compare two materials side-by-side, with useful bar graphs to illustrate the differences. For instance, take a look at their page which compares 6061 aluminum vs. acetal plastic.
Wolfram Alpha caused a big stir when it launched almost exactly a year ago. While it's not the omniscient machine that many had hoped for, it does have a lot of great information about a variety of metal alloys, as well as polymers.
I did find myself struggling a bit at first, trying to figure out how to look up the properties of various alloys. The best approach I found is to begin by entering the general type of thing you're looking for like "aluminum alloy". Wolfram Alpha will then let you choose from a list of various aluminum alloys. From that list, you can find out that Wolfram Alpha refers to 6061 T6 aluminum alloy as "AA6061-T6", in case you want to look it up again later.