For plastic properties only, a comprehensive site is IDES Prospector.
A useful site when I need to make quick rough comparisons between materials is McMaster Carr. McMaster Carr is a large vendor (I highly recommend them). To use their web site to obtain material properties in the find producst box type in your material: steel, aluminum, plastic, copper, etc.... A list of choices for products which include the material in their name will pop up. Near the top of the list will be an item title "About...." Click on the About link and you will get summary info on the material.
For example, if I am interested in stainless steels I type in stainless steel in the find products box. I list of links pops up. The list includes items such as stainless steel flat washers, stainless steel chain, etc.... The second item on the list is "About Stainless Steel." Clicking on "About Stainless Steel" brings up a page comparing the properties of different grades of stainless steels. It includes comparisons of formability, weldability, corrosion resistance, machinability. It also has graphs of yield and hardness for the different grades of steel.
Example of graph of yield strengths from McMaster Carr.
McMaster Carr does not have the extensive databases that the other two sites have but is is useful for quick comparisons.