seeing as this forums is for tut and common question and answer I belive it deserves a thread to the VERC collective.
for all who are unfamiliar with VERC their website is here, http://collective.valve-erc.com/. Its a large collection of tutorials and articles on how to create items for the HL engine, it covers stuff like coding modeling and of course mapping.
I'm suggesting this thread as a good way to keep track of really handy articles for DOD. anyone new to mapping should bookmark the VERC site in a second.

so I'll start it off.

Weather effects (http://collective.valve-erc.com/index.php?doc=1052078644-91336100)
Usable+destructable mortar (http://collective.valve-erc.com/index.php?doc=1047766389-75965200)
Territory control map (http://collective.valve-erc.com/index.php?doc=1026191087-63290200)
DOD gun penatration (http://collective.valve-erc.com/index.php?doc=1056778065-89116400)

GG Shellshock, -inw-, AB... That's Great.

PS: The best mapping link ever: http://www.slackiller.com/tommy14/cs_maplinks.htm ;) It is already in the mapping faq but it is so nice.