I recently got AnnounceRare and had the same issue others have reported with chat commands mysteriously not working. Typing /rare or /rare help caused the standard WoW message for an invalid command: "Type '/help' for a listing of a few commands."

Disabling all addons except AnnounceRares allowed its chat commands to work properly, so I went through and tried to find what other addons were causing the conflict. I narrowed it down to Rarity.

I confirmed that Rarity registers /rare as an alias for its default command, /rarity in AceConsole. I think commands get registered the order addons are loaded, so Rarity registering its function to /rare overwrites AnnounceRares' function. Strangely, /rarity actually doesn't do anything, so that must be why /rare isn't functional either; Rarity's chat command function is broken. People would have instantly identified the source of this issue ages ago if the Rarity options menu popped up instead of nothing happening!

On my system, I changed AnnounceRare's AceConsole registration from "rare" to "annr" and now it works properly with Rarity enabled. A proper solution would be to add an alias for AnnounceRare's chat command we can use in case of conflicts, though that would take a tiny bit of refactoring from how it's currently written (anonymous function).