1. Flash
Useful for product photography (perhaps get 2), outdoor portraits (as fill light), indoor portraits (better get 2), more...
I use Canon, Metz and Yongnuo's (The Yongnuo YN568EX is a bargain, also works with TTL radio triggers, got 2 of em)
2. Radio trigger for flash
A lot of time you wanna shoot the flash off-camera, infrared triggering sucks, cables are complicated...
PocketWizard, Phottix Odin, or if you are on a budget -> Yongnuo YN-622C (Cheap and had no real problems so far)
3. Steady tripod
Basically you can decide between A: Heavy, B: Expensive, C: Cheap and Shi##y
I swear on Gitzo (aka. lasts a lifetime, and you can beat up people with it) but if you wanna spend less, search for Triopo at ebay.
Triopo makes some very good Gitzo knock-offs (I got both, and the Triopo delivers 3/4 the performance for 1/3 of the price, good stuff)
4. Remote trigger for the camera
There are simple ones (focus and shoot) and fancy ones (with interval shooting, EV or focus series, light / sound / motion triggering)
I got a DIY one (search SmaTrig) which can do more stuff than you will ever need, there are radio triggers as well...
5. Primes
Zooms are convinient, but primes deliver better performance (better image quality and much lower aperture) for the money.
A 50mm prime is a good start, its versatile and relatively cheap...
Unfortunately I cant recommend any 50 currently sold, they all have their shortcomings,
but as starter the 50 f/1.8 is perhaps a good idea, it's not good but it's dirt cheap.
You can also get good 50's for cheap (and awesome for moderate price) when going for old manual ones.
You have to know what to look for (I can help) and you will need a adapter to mount em on your camera.
If you are willing to spend more, Sigmas 35mm f/1.4 is great (as you are on Crop it makes a good standard lens)
The Canon 135mm f/2 is great as well (in case you wanna do tele'ish photos... zoo, concerts, whatever)
Canons 85 f/1.8, while not performing like the lenses mentioned above,
is a nice portrait lens for moderate price.
I don't have too much experience with super-tele and wide-angle primes,
so I cant help here ^^;
6. Zooms
Versatile, but they trade that for image quality and speed (aperture)
Super zooms (Zooms with wide range of focal length) gernerally offer poor image quality, try to split ranges!
I usually go with 2 zooms, a wide angle -> low tele (ie 15-85) and low tele -> upper tele (ie. 70-300).
Zooms I can recommend: EF 70-200 L (any) / EF70-300 L / EF-S 15-85 (not the newer 17-85).
7. Filters & Misc
Don't use protection filters (image quality is degraded), use lensshades for collision/bump protection instead (even when there is no sun).
Get some good quality ND filters for your portrait primes (when shooting wide open in broad daylight you often hit the highest shutter speed -> overexposure)