Here are a few good L4D Map Links...

Couple of good maps identified by Jerry:
DeadCity: http://www.l4dmaps.com/details.php?file=70

Resident Evil: Last Escape: http://www.l4dmaps.com/details.php?file=48
Note: Resident Evil: Last Escape is in beta... I had to click on the "changelog" link next to the unreleased version number.
http://www.l4dmaps.com/details.php?file=48&cmd=changelog

Some high scoring multi map campaigns:
Heaven Can Wait: http://www.l4dmaps.com/details.php?file=177

Death Aboard: http://www.l4dmaps.com/details.php?file=33

Decent single maps:
Dam it!: http://www.l4dmaps.com/details.php?file=896