Best tools for determining browser issues?

Just wondering what tools are out there to help identify browser and compatibility issues.

I have a few users that are experiencing serious display problems on these two sites: https://17lizann.info and https://268broadway.info and I can’t figure it out.

Several users on Win7 with IE9 can’t even display the content – just the header. Once on a sub page, they can’t see the menu.

For another user, the thumbnails on the “photos” page first were all squished together (fixed that by removing the Mortgage Calc plugins) and now they are wrapping to the next line instead of showing 5 in a row.

This is a multisite install. I have other sites that use multi-site with other themes and don’t have these problems, but I can’t tell if it is a theme issue or not.

Thanks!