What are the best Nikon D90 accessories that every amateur photographer must have?

I'm an amateur photographer. I have a Nikon D90 with it's kit lens AF-S DX NIKKOR 18-105mm f/3.5-5.6G ED VR. Recently I bought the SB-910 flash. I'm thinking of buying a set of different lenses and other accessories now. Please be professional and concise in answering the question.citing sources is preferable.

You must have those accessories that you need to take the photographs you want to take!

At some point you will say "If only I had a… I could have got a really good shot" That is the time to buy the…

Photography isn't about collecting complete sets of gear - its about have the right gear for the job.

A good bag for carrying and storage. This is mine.
http://tamrac.com/5606.htm

Tripod and a working knowledge of your camera - that's penty to start.

Accessories are a matter of preference and need for your photography style, so what works for me may not work for you.

However, the lenses I have for my D90 are:

- Nikon 10.5mm f/2.8 fisheye (just for fun).
- Tokina 11-16mm f/2.8 super wide angle (landscape and building interiors).
- Nikon 18-200mm f/3.5~5.6 all around lens (travel lens - lighter to carry).
- Nikon 40mm f/2.8 micro (used for closeup more than macro).
- Nikon 50mm f/1.8 (low light).
- Nikon 80-200mm f/2.8 (sports and action photography and serious portraiture).
- Tokina 100mm f.2.8 macro (strictly macro use).
- Tokina 80-400mm f/4~5.6 (wildlife and birding).

In addition, I have the following flash equipment:

- Nikon SB700
- Nikon R1 Close Up Set

And finally, I consider the following essential accessories:

- and 16 neutral density filter - 77mm with step down rings for the various lenses.
- and 400 neutral density filterr - 77mm with step down rings for the various lenses.
- Circular Polarizer for each lens size as they are too hard to use with step down rings
- Wired and wireless remotes and intervalometer.
- Heavy duty tripod and monopod.
- Black Rapid camera sling.
- Think Tank Camera bag
- iMac with Lightroom

Buy a good book in basic photography first. That would be the best accessory every newbie should have. Hold your horses on buying anything at all unless you have understood and applied everything you learned in the book. Lenses and accessories will differ from photographer to photographer varying greatly mostly depending on the subject matter the person loves or needs to shoot. Some will feel complete with just a single fast normal/standard lens and a huge flash but others will go to great lengths in setting up the best artificial lighting fixtures money can buy. Learn photography first before you buy to avoid overspending on things you really won't need in your shooting.