What accessories do you use with your Nikon DSLr?

I'm planning to get a Nikon D3100 and it comes with two lenses. My question is what kind of accessories do people usually buy to go along with it? I'm not too familiar with camera lingo and such, but I do know they put a 'crown' sort of thing on the lenses and that helps to block against sunlight hitting the lenses directly, right? What other accessories do you use? Also, what kind of bag would be good that has two-three lenses holder too? Thanks!

Added (1). @fhotoace: What brand of backpack is it? Can't seem to find a good one haha

* I use a battery grip (not available for the D3100 and not needed on the D2 and D4
* I also have a Nikon external flash used for flash fill and have lens hoods and UV filters for all my lenses.
* I have a GPS attachment, remote shutter release, monopod and tripod with three way head.
* Of course I have a whole bunch of memory cards, 6 - 2 gb cards I use when shooting in the studio, 8 - 4 gb cards used most of the time 4 - 8 gb cards I use when in dusty or wet areas when I don't want to be changing cards in the middle of a shoot and one 16 gb card that was a gift
* My camera case is actually a backpack which will hold two dSLR bodies, four or five lenses depending on which ones and my Notebook computer with all the charging equipment I need when on assignment. I can also get a 1 tb external hard drive in the backpack.