For developing Flash / ActionScript packages on Windows, there is no better free tool than FlashDevelop.
Unfortunately, its reliance on the .Net framework make it ill-suited for AS3 development on Ubuntu. (My attempts to get it working with Mono failed – the installer wouldn’t even work.)
Is there a similar tool for use with Ubuntu?
I don’t think there any other free decent IDE’s.
You could try FDT – a plugin for eclipse, although it is not free. There is an install guide here: http://varga-multimedia.com/2010/02/07/how-to-install-fdt-on-ubuntu/