I've been using MCEdit for ages. It got a bad rap because there was a time when it didn't support the extra block IDs, so IDs above the vanilla range would be replaced with stuff from the Vanilla game. As far as I could tell it just dropped the extra digits (IE block 4095 would become wither 4 or 40 depending on what was available). It was updated a while ago to accept the increased block IDs. MCEdit now supports all the blocks Forge goes. As mentioned though it only has vanilla textures, so most things added by mods show up as purple "Future Block!". However that doesn't stop the user from interacting with those blocks.
If you want to play around with blocks that are added by mods, you'll have to get yourself a sheet of paper and go through in game making yourself a list of blocks and their IDs (including metadata ex: 345:6). You'll be able to use all the tools in MCEdit, but will have to input the block's ID instead of it's name like you can with vanilla blocks. It works perfectly, but might get a little hard visualizing what you're creating because everything is purple.
Single player commands is basically MCedit but in game. You have access to a lot of the same features, but because it's in game you can see the results right away. There's a higher risk of doing something that overloads and crashes your game though. I'm also unsure whether or not it'll work in multiplayer (if it does, you might end up giving everyone access to it).
And as far as I know World Edit is Single Player commands for SMP.
As I mentioned the issue with all of these was that Forge added the ability to use Block IDs up to 4095, which these mods didn't recognize (there was no reason for them to). They've all been updated ages ago though.
