Yes, the Curse website (and Curse Client) is safe to download addons with.
However, like all other sites that have ads (advertisements), you can be infected with viruses from the technology that is used for the ads (specially flash ads). This is mostly true for any site that has ads, Yahoo, MSN, MMO-Champion, and others.
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There are two major sites that deal with World of Warcraft addons. Curse and WoWInterface (See related links for the addresses). Google code is starting to have more and more addons as well, however, those are a little harder to find.
ND Curse of Blackmoor Manor, and other games, can be downloaded directly from the Her Interactive website, by paying money. There's no legal way I know of to download it free, however.
It is against the rules of WikiAnswers website to use curse words.
Curse is not a TM.
You fight grumble about 2 to 5 times and it will drop grumble curse