Why are some of my fonts italic? Which ones are corrupt? Can you help?

You either removed fonts the system needs, the font cache files are goofed up, or both.

Simpler thing first, just clear the font cache data.

Close all running applications. From an administrator account, open the Terminal app and enter the following command. You can also copy/paste it from here into the Terminal window:

sudo atsutil databases -remove

Terminal will then ask for your admin password. As you type, it will not show anything, so be sure to enter it correctly.

This removes all font cache files. Both for the system and the current user font cache files. After running the command, close Terminal and immediately restart your Mac.

If that doesn't change anything, then you likely removed Arial regular and whatever else from the hard drive. You'll have to restore them. If you have a backup, just copy the missing fonts from there, placing them back into the same folder. You can also copy them from another Mac. Preferably, one running the same version of OS X, but it can be pretty much any other recent version.

If you have no access to the missing fonts otherwise, you'll have to restore them from the Snow Leopard DVD. Follow the instructions at the bottom of Font Management in OS X.

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Ok, I now have arial black.ttf - where do I put it? I do have a LIBRARY folder... and then?

Arial Black belongs in the main /Library/Fonts/ folder (the Library you see alongside the Applications and System folders). When you drag it in there, the OS may ask for your admin password before allowing the copy or move to complete, but I don't think it does in Snow Leopard.

Repeat for any other fonts you need to copy back in. In the same article I linked to above, you can check the first section out to see what fonts at minimum should be in the /Library/Fonts/ folder.