How can I view/edit Minecraft's level.dat file?

Minecraft's data files are in a proprietary file format called NBT (named binary tag).

You're gonna need an editor specifically designed to open these, such as NBTExplorer (runs on Windows, Linux, Mac).

There is also the appropriately named NBTedit, but apparently it is no longer being updated and the author himself recommends NBTExplorer.

You can open it with WinRAR or Windows Explorer, then edit the file inside with Wordpad, Notepad, or any other program.

The file contains the /gamerule's and the properties like: Cheats, your Seed, the directory of all the world files, the game mode, and more.

But I have not edited it this way though, I just viewed it. So if it breaks your world, then it's not my fault.

So the best way to edit it is NBTExplorer.

The link provided by ShadowPlays is now broken, but the tool it was leading to is still around. It's called WebNBT, and can (at this point) be found here:

https://irath96.github.io/webNBT/

If the link doesn't work for you, try googling webnbt, that's how I found the new link