Tired of me talking about Acorn yet? Too bad!
Jens Ayton has written a plugin for Acorn that adds an Export menu item, which will let you save the current document as BMP, GIF, JPEG, JPEG 2000, OpenEXR, PDF, Photoshop, PICT, PNG, SGI, TGA or TIFF. It uses Apple's ImageIO routines to do this. Source is also available.
(Acorn already lets you save as some of these file types, but I think the idea was to cover everything that ImageIO offered.)