Lasyk myThemeCreator is a really easy to use app for creating mobile phone themes. It supports over 80 devices, mostly Sony Ericsson and Nokia S40 Handsets.
mTC runs on Windows, Mac OSX and Linux, since it's Java based.
I'll stick to the official tools, but for those who wanted ease of use, give mTC a shot.
Official webpage: http://www.lasyk.net/mTC/