Minecraft 1 7 with auto updating
Same with adventure maps, though I can't guarantee that later releases won't break them.
I'd also like to see some maps that are like replicas of a Minecraft house or city at 8x scale, it would be fun to walk through at 8x size and feel "normal", and then at tiny size to feel microscopic (The "giant house" maps I've seen are usually much greater scale than that.) Here's an example of the kind of video I like to see: Gulliver v0.14.2 only works on Minecraft 1.6.2.
You can adjust your size by pressing the 'R' key to double your current size and the 'F' key to halve your current size.
Potion of Embiggening makes you 4 times your height, Potion of Embiggening II makes you 8 times your height.
Potion of Ensmallening makes you 1/4 your height, Potion of Ensmallening II makes you 1/8 your height.
Basically, for both effects, I'm shooting for about equal parts awesome and annoying. Otherwise you'll be knocked off if you get hurt while riding, or if the ride falls more than a certain distance!
(The classic essay "On Being the Right Size" mentions a number of the ideas that I used when devising how the mechanics would work differently in Gulliver.) Tiny players can ride larger entities by right-clicking on them while holding String. Leave feedback on what you think, or any other suggestions for how Minecraft could work differently when you're tiny/huge.