Add relevant zone information to alarms (so you know where that obscure subzone that is under attack is!),
Use hyperlinks in alarms to easily open the world map to the proper zone and ping the location of an alarm,
Suppress superfluous messages (handling alarm spam),
Custom-color alarm text,
Highlight the faction involved ("the Horde" or "the Alliance"),
Add timestamps,
Shorten WorldDefense and LocalDefense messages in a variety of ways,
Customize alarms' text format (as just one example, you might use "Under attack: <Place>" instead of "<Place> is under attack!"),
Display alarms while in any instance (or optionally, only battlegrounds),
Display ZD messages in any chat frame(s) of your choice,
And more!
Quick Reference
Slash commands:

/zd - Basic settings.
/zd help - Additional commands (such as ping location scanning - see "SCAN FOR ALL PING LOCATIONS" in readme.txt).
/zd adv - Advanced settings.

Change which chat frames ZoneDefense messages appear in:

Right-click on the desired frame's tab and select "Settings" (under the heading "Filters").
Select the "Other" category.
Check (or uncheck) the "ZoneDefense Alarms/Chat" option on the bottom of the list on the right-hand side.
What's New?
Here are just a few highlights from recent updates. See the change log for more.

Options to handle Grizzly Hills PvP quest alarm spam separately from other alarms. (0.80)
Updated for WoW 3.0. (0.70)
Share alarms with other ZD users, letting alarms others share be seen by you while you are in an instance (display disabled by default). (0.54)
Use with Intel
Intel is a sister addon to ZoneDefense. It is available for download here (or at WoWI, here). Neither is required for the other to operate, but they do complement each other in a number of ways:

When ZoneDefense looks for a place to ping on the world map, it checks with Intel to see if one of its icons would work best.
When ZoneDefense receives shared alarms while you are in an instance, it forwards them to Intel. This allows Intel to continue to update world objective information when it normally would be unable to do so. (ZD does not have to be set to display shared alarms to forward them to Intel; it need only be set to share them.)
When Intel outputs a "pseudo-alarm," such as when a Wyvern Camp is captured or lost (an event not reported by LocalDefense/WorldDefense and thus not normally output to the chat log), it can have ZoneDefense format the message in the manner of a LocalDefense alarm, as customized by the player.