MC Edit is the best for spawner stuff, coping things and making blocks float, though I personally think world edit is better for editing terrian as it works inside the game rather then as an external tool. You can get it here, though you'll also need forge or some other way of modding it into your game.
For making custom items I think INVedit is the best choice, though offical development has stopped, so you'll need to find a thread where someone has kept updating the source code to work with the latest version of MC. It's good in that it's easy to format text, lore, enchants, armor colour and so on without having to use any commands.
There are also tools you can use to generate /give commands for custom items as well (running a quick search found this), which are a little more complcated (not too much though :P) and allow you to add attributes as well.
I don't know of a better tool to use for custom mobs/spawners then MCedit though. If anyone has a good one I'd be quite interested as well.
Wanna know a cool tip? If you select text and click the link symbol, you can hyperlink something to a webpage without displaying a really long address. If you over one the address will appear in the bottom left.