A 500mm IS L lens is a must-have. You simply can't take decent photos without one.
A shoe-mounted flash is a great accessory, but takes some time to master properly. However once you do you'll hardly ever take it off!
I find the battery packs make an enormous positive difference in handling, but others find them cumbersome.
Re your Sigma post: I can't add anything really useful, but stay away from cheap super-zooms. Or any super zoom for that matter.
As others have noted Sigma make some great lenses, particularly their Macros. AS good as most of their lenses are, Canon (and Nikon) also make some real duds, so check the reviews.
Photo forums like POTN, fredmiranda.com and dpreview. All have more info than you could possibly use in a lifetime and are well worth joining. POTN and fredmiranda have buy & sell sections where gear can be had for much less than retail - although sometimes you really have to work to get them to send it to Australia.
Happy Snapping!