Dota-Allstars 6.49 Quest
Ok, i'm sure this quest is pretty famous and most of you already know. In this version, we search the Frog within Ice (IceFrog). Each frog carried a letter that will form Invoker Skill (Quas, Wex and Exort). After we find and kill frogs that it's letter form a skill, a portal will be opened and we will fight with a boss for each skill (3 hidden boss). After we beat them all, Kael the Invoker will come and fight you. The only hero can beat him single handedly is Ursa, no one else could. Check this link for more details. You can also watch the video about this quest.
Dota-Allstars 6.39 Quest
IceFrog is hiding plenty of Frog (about 20-22) and named them with the Beta Tester nick name as a thanks. You can check the official thread of this quest here.
Dota-Allstars 6.29 Quest
Well, it's very old and i can't find much. But you can check it yourself in this video :)
So, how is the chance for this tradition to continue? Pretty big actually. If you forgot, here is a quote from IceFrog interview.
Q: Are you going to do any mystery quests like the previous Invoker quest?
A: I can't say when I'll do it, because it would ruin the surprise, but I do try to do some fun side quests every now and then. You can expect one in the next few versions ;)
"You can expect one in the next few versions" -> Well, he said this on Dota 6.57b. And Dota 6.59 is indeed a few version after that. Ok, i think my hope on Dota-Allstars 6.59 quest is going up! :D
Update: It's here! Check Dota 6.59b for the quest and share your find on Dota 6.59b Quest Guide!