Hmm.. ok.. lets see:
Worth Every Penny
Nikon D5200. At first I wasn't convinced that it would really be worth it since I already had the D5100 - but the difference in IQ between the 24 mp sensor and the 16 mp sensor was truly astounding. Everything else was just a bonus - like the better AF, etc.
Sigma 70-200 mm F/2.8 OS - I bought this shortly after getting into photography - at the time I had a used D5100 and a couple of lenses, and the thought of spending $700+ on a single lens was a real mind blowing notion for me, but eventually I pulled the trigger and bought it. Probably one of the best investments I ever made.
Black Rapid Strap - Both the RS-5 and then eventually the Yeti. Again, when I started the thought of spending $75+ on a camera strap seemed pretty ridiculous, but after getting one I realized even that first day it was worth it and more.
Sigma 18-50 mm F/2.8-4.5 DC OS HSM - I wanted something wider angle and originally I was looking at a lens in this category with a constant 2.8, but after thinking it over for a while I realized that for most of what I'd use the lens for, I'd be stopping it down higher than 4.5 about 90+ percent of the time anyway, so I went with the much much cheaper variable aperture version. It's been a fantastic addition to my kit, I have the 50 mm 1.8 AFS-G when I want a wider aperture, and this one is a big step up from the kit lens in optical quality so best of both worlds really.
Nikon 85 mm 1.8 AFS-G - it was a bit expensive for me at the time, but it's turned out to be a fantastic lens. With two cameras when I'm shooting an event I can put the 85 on one camera and the 50 on the other, and I've got fast lightweight glass that gives me both great portraits and a medium range telephoto without having to lug around anything heavy.
Adobe Photoshop - Pretty much a given I think
WU-1A WiFi adapter - I use this when I'm shooting events, I can throw it on the camera and then preview the images on my tablet rather than having to wait till I get home to see them on a larger screen. Lets me see things like missed focus that is hard to see on the camera LCD, and that way I can do a reshoot if needed while I'm still on site. Also pretty much negated the need to think about an upgrade to the D5300.
Yearly membership to the Henry Doorly Zoo - absolutely no question the best $80 I ever spent.
Sigma 2x APO Teleconverter - It doesn't get a ton of use, but in a situation where I need a little extra reach when used in conjunction with the 70-200 mm F/2.8 it has been a really handy item to have in the bag. Sort of gives me the best of both worlds, I've got a good fast zoom if I need it and if I need something with a bit more range I can slap on the TC - and it's a lot easier to carry around that an another big heavy telephoto lens.
TPF Membership - Ok, granted I could have probably gotten by without this one but I like being able to change my tagline, and really I'm on here enough and learned so much from the place that I'd feel really bad if I didn't give them a bit of financial support.
Waste of Money:
UV Filters - When I first got the D5100 one of my lenses came with a UV filter, the other I bought used and bought a UV filter for it. I read here that the UV filter would decrease IQ and sharpness but after testing it on the D5100, honestly I didn't see much difference. I did the same test later on with the 5200 - and boy, did I see a difference. So the UV filters came off and went into the bag. Haven't used them since, I'm only hanging onto them because I already have enough coasters and if ever do decide to go out shooting in a tornado I might actually throw them on again for that one scenario.
Sigma 1.4x TC - I bought both this and the 2x at the same time, just haven't found a whole lot of use for the 1.4x at this point. I've used it a few times, but in almost all cases I can crop the image sufficiently without it that it just really isn't all that useful. I got more use out of it with the D5100, but since I got the D5200 it just sits in the bag and collects dust for the most part.
Cheap camera straps - I tried a couple of the cheap, off brand made in China camera straps at one point - they worked I guess, but they were pretty uncomfortable with the heavy lens attached after wearing them for a few hours. The black rapids were definitely a huge step up and I wouldn't think about going back.
Cheap monopod - I don't use the monopod a lot, usually only in the fall/winter when I'm at the high school shooting a sporting event. It's nice to have for that because it's easy to deploy while in the bleachers, keeps the camera pointed in the direction of the action so all I have to do is track and shoot, and it doesn't take up a ton of room so it doesn't bother the people around me like a tripod would. It worked ok at first, but the locking mechanisms get loose now after only a few uses and I have to continuously tighten them down with an allen wrench, which is kind of a pain. I'll most likely be getting a much higher quality one soon, but since I don't use it a ton it hasn't made it to the top of the priority list just yet.
Rubber Lens Hood - I got one for the 70-200 mm thinking that it would be nice to have for shooting outdoors, the thought was I could put a CPL on and be able to adjust it easily without having to take the hood off and on, etc. This might just be me but I hated the thing. After trying it a couple weekends in a row I decided it was a lot less hassle taking the plastic hood on and off to adjust the CPL, or just leaving the plastic hood off when I used the CPL.