Type "Bug 1234" in bugzilla comment text and when viewing bug details, it is automagically linked to the bug. If the bug is closed, there is a strike through!
It is not limited to just "Bug #". The feature extends to attachments, comments, urls etc. More details in the link to bugzilla documentation.
http://www.bugzilla.org/docs/tip/html/hintsandtips.html
A very handy feature some bugzilla users may not know about!