As I was browsing the WoW forums earlier, I came across this little gem of a thread. A Druid named Scarmbrena from Boulderfist managed to obtain over 122,000 health (yes, you read that right) by stacking a buff called Mol'dar's Moxie.
Mol'dar's Moxie, for those that don't remember the good old days of Dire Maul North "Tribute runs," was a buff given to you by Guard Mol'dar that grants an additional 15% stamina for 2 hours.
Scarmbrena describes in the thread how she was able to stack this buff a total of 15 times, bringing her total stamina to 11,896 and her health to a whopping 122,394! Just to put that in perspective, Pathaleon the Calculator (the last boss in the Mechanar), has 130,000 health.
Obviously, this qualifies as an abuse of game mechanics (albeit a creative one), but even more surprising is that the patch notes for 2.1 state that the Dire Maul Tribute buffs shouldn't work on a player over level 63, yet in the screenshot linked in the thread, Scarmbrena is level 70.
I can't say I recommend that anyone try this since Scarmbrena almost got banned herself for it, but it is interesting to see a player amass that much health!