Although it is dependent on Modernizr and jQuery, Webshims Lib is incredible!
Use this to shim anything that isn’t supported:
Or something like this to only support certain features:
Note: The demo in the article link above did not initially work for me in IE<9, but that was because, not being my normal browser, my IE did not have Flash installed… :-) Worked quite nicely once I had the right stuff installed! Perhaps some form of "plug-in not found" message would be nice, instead of, you know, nothing…