The guys who created Check Server Headers Tool - HTTP Status Codes Checker have created an expanded version called URI Valet.
It's a silly name, but it's a very nice too for viewing your HTTP headers. You can pick the User Agent you want to validate as and you can also set the If-Modified-Since header.
In addition to showing the header values, the tools also shows you the objects loaded by each page in tabular and pie chart views and calculates estimated download times from those objects.
It even has convenient links to W3's validator tools.
This is one to bookmark.