Adobe currently offers a free plug-in to export artwork to GIF89a. Version 4 of Painter has built-in tools to help create graphics for the web.
Transparency is designated by using a mask and a threshold setting.
See chapter 7 of the Tutorial manual for more details. GIF89a plug-in put it on this list. I have no idea if this is different from the Photoshop version, but you can find out for yourself. If you want to see product info added to this list, click on the Comments link below.