Spectator Mode is a powerful noclip mode useful for zooming around a world really fast as well as teleporting to other players, although you can't interact with any inventories or generate loot chests. However I don't see a way to select it.

/gamemode spectator I believe. GM 0 is survival, GM 1 is creative, GM 2 adventure, logically GM 3 would be a spectator but I don't see this number on any sites so you might have to type it all out. of course this is the stock Minecraft way, I know there are 4 modes on the mode button but yes, I believe it's adventure and adventure+ Creative and Survival... it is missing the spectator mode...

/gamemode sp also works. I was hoping to see it in the button toggle though as the mod does everything else I can imagine right, like showing mushroom island biome squares as safe regardless of light level and giving the old NEI behavior when taking items from the right side.