This page contains a short overview of all world bosses in game. These bosses are considered raid bosses since it takes more than a party of 5 players to down them. Some people have roughly estimated their spawn rates:
Initial spawn:
-Tuesday Night, very late (~1-5%)
-Wednesday Night (50-80%)
-Thursday Night (15-20%)
Respawn:
-2 days after previous kill (30%)
-3 days after previous kill (40%)
-4 days after previous kill (30%)
Server crashing or going down counts as that world boss having been killed, and starts its respawn timer
At the time of the testing, Azuregos/Lord Kazzak used to spawn a little earlier than the world dragons. The dragons spawn one unique per the 4 spawn locations, typically all at the same time. One continent being down without the other desyncs the dragons, possibly making them spawn duplicates. (Thus occasionally spawning 2 of the exact same bosses in different locations)
History of World Raid Bosses
Originally, Azuregos and Lord Kazzak were added early in WoW. They were one of WoW's greatest promotions of guild vs. guild rivalry, as killing them was a matter of a combination of tracking their spawn, forming a raid, and killing them while beating the encounter AND the opposing faction (and at times same faction griefers). Their loot was also among the best at the time of their popularity.
Shortly before the Ahn'Qiraj event, the four Dragons of Nightmare were created, and added an entire new round of World Raid bosses. They were initially killed for having the best items for their slots, and later farmed for their nature resistant gear when it became apparent that some Temple of Ahn'Qiraj encounters required it. They were also involved in the Scepter of the Shifting Sands green piece questline.
As time passed, and Naxxramas came out without accompanying world raid bosses, the popularity of the world raid bosses began to fade.
With patch 2.0, Highlord Kruul began to terrorize Azeroth, in place of Lord Kazzak (and his Azeroth loot table).
After the Burning Crusade came out, two new world raid bosses were introduced, Doom Lord Kazzak and Doomwalker (there were others initially shown in databases as ?? bosses, including a rare spawn from a quest in Blade's Edge in which you can spawn a raid boss who flies away, but is unaggro-able. But currently it is accepted that there are 2 BC world raid bosses). The popularity of the two was short-lived. After a couple months it became apparent that loot off of Karazhan was on par or better, while not having the additional faction vs. faction griefing factor.
In BC WoW 2.2, world bosses were often left untouched for long periods of time. There were occasional guilds that use them as either practice for raiding, or undergeared guilds trying to gear up. But with the announcement by a blue that they did not plan to add additional world raid bosses (source?), the days of a world raid boss came to an end.
When WOTLK came out they never added any world raid bosses. Now the bosses that are still around lay untouched. It is the end of world bosses...or is it. In Cataclysm they plan on taking the game back a bit to what it was like when it first came out. That may very well mean that we may not have seen the last of the world bosses. It is very likely that we have though. The world bosses are very inpractable.
Dragons of Nightmare
Emeriss
Emeriss
Location: The Great Trees
Raid 40
One of the Four Dragons corrupted by the Nightmare, Emeriss is a gruesome, rotting monstrosity. Along with his AoE Sleep, he also casts a disease which damages nearby allies, and turns the corpses of the dead into deadly mushrooms. And every 25%, he casts Corruption of Earth, a Shadow DoT that deals 100% damage to the players health over 8 seconds.
Lethon
Lethon
Location: The Great Trees
Raid 40
One of the Four Dragons corrupted by the Nightmare, Lethon has become exceedingly evil, enough so to darken the hue of his scales, and his eyes have also been forced open, an oddity amongst Green Dragons. He has a massive Shadowbolt Volley, and every 25% he extracts shades from his enemy that will heal him.
Ysondre
Ysondre
Location: The Great Trees
Raid 40
One of the Four Dragons corrupted by the Nightmare, Ysondre's once beneficial healing powers have given way to dark magics. Casts an AoE Lightning Wave that deals direct Nature damage, and summons fiendish Druids every 25%.
Taerar
Taerar
Location: The Great Trees
Raid 40
One of the Four Dragons corrupted by the Nightmare, Taerar was perhaps the most affected of Ysera's lieutenants. His corporeal form has been shattered and he exists as a specter. Has an AoE Fear, and creates 3 Shades of himself every 25%.
Others - Eastern Kingdoms and Kalimdor
Azuregos
Azuregos
Location: Azshara
Raid 40
A mighty blue dragon who resides in the wilderness of Azshara, perusing over the countless magical artifacts there. He randomly teleports himself and all players within 30 feet a short distance, wiping aggro, casts a blizzard-like spell that eats mana, and occasionally puts up a magic reflection shield. Also, in order to prevent graveyard-zerging, he places a debuff on players that he kills. Any living players with this debuff will freeze whenever they go near him.
Highlord Kruul
Highlord Kruul
Location: Blasted Lands
Raid 40
Highlord Kruul is a doomguard boss who appears in various parts of Azeroth during the Dark Portal Opens event, introduced in patch 2.0.3. After the Dark Portal reopened, he officially replaced Lord Kazzak (now Doom Lord Kazzak in Outland's Hellfire Peninsula) as the Burning Legion's field commander in Azeroth, residing in the Tainted Scar in the Blasted Lands, southwest of the Portal.
He has since been removed from the game as of the release of The Burning Crusade.
Teremus the Devourer
Teremus the Devourer
Location: Blasted Lands
Raid 10?
Teremus is an immensely powerful black drake found in the Blasted Lands. He has an AoE Lifesteal, and wipes aggro when he uses a knockback attack. When he gets down to 20% health, he regenerates to full health, making him one of the most difficult and prolonged fights in the game.
He used to be a popular target for the makers of machinima, most frequently being kited by a hunter and a priest from the Blasted Lands to Stormwind or Ironforge, but as of some past patches, he is now chained to the Blasted Lands and will leash back when a player tries to kite him out of the zone.
Event/Quest - Eastern Kingdoms and Kalimdor
Omen
Omen
Location: Moonglade
Raid 40
Omen is a demon boss summoned in the Moonglade during the Lunar Festival.
Others - Outland
Doom Lord Kazzak
Doom Lord Kazzak
Location: 
Hellfire Peninsula
Raid 25
Former commander of the Burning Legion forces in the Blasted Lands, the former-Doomguard, now crowned Doom Lord Kazzak awaits with his formidable troops for the heroes of Azeroth to challenge his reign as Commander of the Burning Legion in Outland. He consumes the soul of all beings he slays to heal himself, casts a curse on players that makes their mana pools combust and if he isn't killed in 3 minutes, goes into his "supreme mode," allowing him to completely obliterate everything in his path.
Doomwalker
Doomwalker
Location: Shadowmoon Valley - In front of The Black Temple
Raid 25
Doomwalker is a powerful Fel Reaver sent by Kil'jaeden to assault the gates of the Black Temple in Shadowmoon Valley. He's a clear reminder for Illidan that Kil'jaeden hasn't forgotten about him...
Template:Raid encounters