It wasn't exactly the Death Star...

...but we have had a space station in orbit around the Earth that had its own main cannon, making it the only spacecraft to date with weapons mounted on it:

http://articles.techrepublic.com.com/5100-10881-6170974.html?tag=nl.e101

(Well, the only one that we have any declassified evidence of to date. But still, how cool is that? Seriously!)