As of WoW 2.4 AnduinLothar will no longer be maintaining DamageMeters.

For more information see the Cosmos UI Forum Thread: http://www.cosmosui.org/showthread.php?t=57523

Maintainers:
Up to 5.8.3 was AnduinLothar
Up to version 5.0.0 was Dandelion

THIS VERSION IS ONLY LOCALIZED FOR ENGLISH AND GERMAN.
Feel free to contact me if you want to try to localize it for another language. There used to be a French one, but it's kaput. Bellow is an old Chinese version I found.
DAMAGEMETERS 5.0 (CN) : http://bbs.game.mop.com/attachment.php?aid=96455 (I guess)

Summary:
DamageMeters monitors damage and healing done by and to yourself and other nearby players. It keeps a running total for each player and displays the information as a bars chart in a frame on the screen. It is perfect for real-time comparison of the various members of your party or raid. At any time the text results can be output as "says", to raid or party chat, or whispered to another player. Many console commands exist for customizing the meters: type /dmhelp for a listing.

FAQ:

Q: Why the "-AL"?
A: Dandelion, the orig author, has retired from WoW. AnduinLothar (me) modified v5.0.0 from Cosmos because that version works with English, German and French clients. AnduinLothar has no long term plans to enhance functionality and would gladly hand over maintinence to someone else, but decided DamageMeters had a large enough user base that it deserved to be updated to use the 1.12 addon communication API.

Q: MY DM WINDOW DISSAPPEARED!
A: Use /dm resetpos. This should make the DM frame visible and put it against the right side of the screen.

Q: What happened to the "Reset Each Combat" option?
A: It was made irrelevant. DamageMeters now keeps in essence two full sets of information: one normal, and one set that applies only to the latest fight. This second set, the Fight set, is what provides the information for the DPS, HPS, DTPS, and HTPS quantities. Those quantities are basically the normal quantities but divided by the duration of the fight. If you turn off the "Visible Quantity\Show Fight Data As Per/Second" option you have what is in essence the same thing as what you had before with the "Reset Each Combat" option.

Q: Why can't DM just automatically sync?
A: v5.1.0 has an auto-sync option. This will sync when you lave combat once every three (3) minues. In previous versions this was left out for the following reasons: Primarily because sync'ing takes a long time. There is just a lot of data to transmit, and since WoW likes to boot clients that send data too quickly DM transmits it slowly. If you are having problems with frame lag or being dissconnected randomly you should dissable auto-sync and do it manually after boss fights.

Q: How do I get other player's pets to show up?
A: As of v5.8.0 "Treat Pet Data As Your Data" now adds a player's pet damage to their damage. This used to only work for the current player. Now it works for party and raid members' pets as well.
Also as of v5.8.1 pets will now be included with "Only Monitor Group Members" enabled.

Known Group Pet Exception Cases:

If more than one unnamed pet of the same type (e.g. Water and Fire Elementals) exists at the same time the events are all attributed to the same entity. If "Treat Pet Data As Your Data" is enabled the event data will be attributed to the last person to be scanned (highest raid ID).
If there is a duplicate name among pets and group members the group member will be attributed all the events associated with that name. The only exception to this is if the conflict is with you or your own pet, in which case the events should be attributed correctly, because they happen on different events.

Q: Why don't Earth Shield, Judgement of Light, Prayer of Mending, Life Bloom, etc get attributed to the right caster?
A: These heal spells all say that they are cast by the target. ex: "Bob's Earth Shield heals Bob for X" which means there's no way to tell who actually cast the spell. Theoretically you could attribute it correctly for most 5 mans where you don't often have duplicate classes, but in raids where you have people casting the same spell and can even have multiple healing spells up t the same time there's no way to tell who to attribute it to.

Q: Does this addon increase combat log range?
A: DamageMeters has an option to extend range to 2000 that is now enabled by default. 2000 yards might actually be larger than the maximum allowed by the game, so it will be set to the max allowable range. It has been tested that setting the range to 2000 allows you to receive log data from anywhere in Arathi Basin while standing at the spawn point. So chances are if you're in a raid fight you'll get all the logs from your raid members.

Q: Why might another player have different meter numbers?
A: Aside from the above reasons, you also stop receiving combat log fight data when you die, other than things like death messages. If you die a lot on trash or you die early in a boss fight your meters will be wrong for that fight. For this reason it is recommended to sync after combat ends so that the person who was alive the longest (who probably has the most accurate data, other than pet classes) can spread it to everyone else.