I can't say from experience using myself, but I have seen some announcements made by RareCoordinator @ Curse.
Seen other announcements, but I never saw a mention of an addon name, so not sure if simply a macro or manual announcements.
I pressed this macro whenever I saw a rare alive before I killed it.
I made a channel (for others that wanted to see when it spawned last time) with the number 5, you can change it to whatever channel number you have. It will say which time and which rare is up in the channel when you press it. It will also say "X rare is up. Time: XX:XX" in general chat so others can come and kill it. The macro creates a DBM timer lasting 60 minutes as well for every rare you use the macro on, which makes it easy to see which rare is coming up next when you have killed them at least once.
The downside with this is that you need to see the rare for it to work, so if you see someone saying X rare killed then it's pretty useless. Basicly need to run around and find all the rares at least once to make it efficient. I knew all the spawns so I guess you might need to look up on those.
2nd vote for RareCoordinator. It synchronizes information with everyone on your server who also uses the addon to know when mobs were last seen\killed and if one is alive it makes a noise and puts a TomTom arrow up pointing you to its location.
Gershuun @ Borean Tundra US - Interface & Macros Moderator
SilverDragon or NPCScan work on a different mechanism, where they will detect the presence of an NPC without the need to target it first, or at least the first time you see it before clearing the cache.
RareCoordinator seems to just communicate the time of the last kill, and the current status if at least one user is targeting it so you can estimate when it should be back up again.