Recently came across this article where a photographer lists a few purchases that completely changed the way he approached photography. Thought it would be interesting to hear from Nikonites on what photography purchase(s) you're in love with / changed the way you shoot / completely blew away your expectations / etc. And for laughs, if you could list one piece of gear that you consider a major letdown or an absolute dog, that would be interesting too 
For me, my favorite purchase thus far would have to be a beater 55 f3.5 Micro-Nikkor P.C. non AI (that I subsequently AI'd) and PK-13 that I picked up used for a song. However, it has easily spent more time on my D7100 than all my lenses put together. A few days after it came in, I was out testing the lens on my D3300 when a dragonfly happened to land right next to me. The rest, as they say, in history! My dumbest photography purchase would have to be a 28 f2.8 AI-s (supposedly great lens that I paid a pretty penny for) for use on DX (I already had a 35 f/1.8 dx and a 28 f/3.5 Nikkor H Ai'd, so it didn't really add much to my kit). For that matter, I usually grab my 18-55 kit lens when my iPhone won't do for shooting wide, so I'm embarrassed to say that even the venerable 35 f/1.8 dx has seen very little use. I'm hoping to change that!
Blue Dasher (female) by p.g.photo.graphy, on Flickr
For me, my favorite purchase thus far would have to be a beater 55 f3.5 Micro-Nikkor P.C. non AI (that I subsequently AI'd) and PK-13 that I picked up used for a song. However, it has easily spent more time on my D7100 than all my lenses put together. A few days after it came in, I was out testing the lens on my D3300 when a dragonfly happened to land right next to me. The rest, as they say, in history! My dumbest photography purchase would have to be a 28 f2.8 AI-s (supposedly great lens that I paid a pretty penny for) for use on DX
(I already had a 35 f/1.8 dx and a 28 f/3.5 Nikkor H Ai'd, so it didn't really add much to my kit). For that matter, I usually grab my 18-55 kit lens when my iPhone won't do for shooting wide, so I'm embarrassed to say that even the venerable 35 f/1.8 dx has seen very little use. I'm hoping to change that!
Blue Dasher (female) by p.g.photo.graphy, on Flickr