I'm working on a web site that is only used internally in the company, and the only way to get to site is in a popup window launched by different web site. Is there any reason I should include a <meta name="description" content="blah blah" /> tag? Besides search engines what other reason is there to have a description meta tag?
For an internal site there is no reason to add a description meta tag. It is indeed for search engine usage, so unless you are using an internal search engine (say a google mini) this is entirely unneeded.
From wikipedia - the description attribute:
The description attribute provides a concise explanation of a Web page's content. This allows the Web page authors to give a more meaningful description for listings than might be displayed if the search engine was unable to automatically create its own description based on the page content.
(emphasis mine)