This may sound a little strange but my suggestion for "should own" accessories is books. After owing a Digital Rebel since 2005. I finally decided to get serious about making it a hobby this past fall. Since then, yes, as you can see from the sig, I've acquired some equipment. But the single biggest thing that has advanced my knowledge and photography is books. Here's what I've read/am reading/have already ordered in the last 3 months:
- Digital Rebel Field Guide
- Digital Rebel XTi for Dummies (they didn't have 300D version that I could find)
- Understanding Exposure
- Understanding Composition
- Photoshop Elements 7 - The missing Manual
- Teach Yourself Visually - Photoshop Elements 7
- Canon EOS 50D Field Guide (ordered but not yet released)
- David Busch's Canon EOS 50D Guide to Digital SLR Photography (ordered but not yet released)
- Canon 50D: From Snapshots to Great Shots (ordered but not yet released)
I know it sounds dry, but I set aside at least an hour per night to read at least a chapter. Sometimes I switch between books if it starts to get tedious. Have your camera sitting next to you so you can try things with it (especially when reading the camera-specific books).
I'll leave the equipment recommendations to those who know what they're talking about. I've acquired a fair amount of equipment in the past 3 months but educating myself about the camera and photography in general has been the biggest thing responsible for any improvement I've had. You may not need to do this; but I certainly did. Good luck!