Here are some things I've recently been thinking of buying.
Presslite Vertex - a hotshoe flash accessory that has positionable mirrors to direct the light into two or three places at once. Price has been reduced at Amazon to less than $35 (used to be $50).
What I hope it will do for me: allow me to let one flash function as two or even three flashes. Although I do have a second flash, I can only hold one flash. With this, it's like I have another flash I can use handheld, and it's even possible to put use my flash on-camera while still retaining a lot of control over its direction.
What's stopping me: it will only be useful indoors where there are suitable bouncing surfaces. These days I don't often take photos indoors. That will likely change when our second baby comes. I'm also deterred by the complexity of the use of the device.
SaxonPC grids - these are economical alternatives to the Honlphoto grids.
What I hope it will do for me: grids are more restrictive than snoots. This will allow me to do more experiments with restricted light.
What's stopping me: it seems easy enough to make a DIY grid from black straws (but it won't be as good looking). [ I haven't found black straws either... :-/ ]
Coco Ring Flash - a cheaper (~$50) ring flash alternative to the Ray Flash (which I like but which I find overpriced). Other cheap ring flash adapters have had terrible reviews but this one looks ok.
What I hope it will do for me: I like the photos that David Hobby made using the ring flash as soft on-axis fill to control the highlight-shadow ratio.
What's stopping me: in theory, an umbrella can provide soft fill as well though it wouldn't be as on-axis as a ring flash unless it's directly behind me, which is quite hard to do handheld :). I'm also wondering how easy it will be to use manual flash for my key (either using the SB-800 with the ring flash on-camera, and the YN-560 as key; or YN-560 with ring flash on-camera and SB-800 on SU-4 mode as key).
Cowboystudio Multi-Disc Kit ($48) - a reflector holder and a large 5-in-1 reflector.
What I hope it will do for me: I don't often get to experiment with reflectors because I find it hard to position them. This will solve that problem.
What's stopping me: this will be useful only for setup shots, and I don't do many of those.
Cowboystudio 24" Softbox ($41) - this is a cheaper alternative to the popular Lastolite Ezybox.
What I hope it will do for me: I know that a softbox has more controlled light than an umbrella, and I have never had a softbox before (closest I have is a brollybox). I am interested in experimenting with softboxes, and this one is affordable enough.
What's stopping me: I don't think it will be easy to use this handheld, even with a monopod.
A fast prime (35 1.8, 50 1.8, or 85 1.8)
What I hope it will do for me: get a really shallow DOF, take low-light photos without flash (I anticipate I can't use flash when our second baby is born).
What's stopping me: 1.8 is only 1 1/3 stop wider than my 2.8, and in return I lose the versatility of the zoom. I'm also torn between the utility of the 35mm but the shallow DOF of the 50mm and 85mm. I'm also wondering whether or not the money is better saved toward my next object of desire:
Sigma 50-150 2.8 or Tokina 50-135 2.8 (discontinued)
I am drooling for a 70-200 2.8 for the range and the shallow DOF, but it's extremely expensive and it's impractically bulky for me. Both of the above lenses are 1.5x-crop lenses with a range similar to the 70-200 on a full frame camera, are far more compact, and far more affordable -- in short, a much more practical alternative to the 70-200.
What I hope it will do for me: get a really shallow DOF, get candid photos of our kid from afar.
What's stopping me: it's definitely much more affordable than a 70-200 but it isn't cheap (at least not for me). I'm also wondering how easy it will be to use it without optical stabilization. The last time I had a lens with a similar reach, I was using a camera with built-in stabilization, the Pentax K100D.
What I hope it will do for me: best bang for the buck for sheer power. Can be used to get soft light (with a modifier) while overpowering the sun.
What's stopping me: it's only useful for setup shots. No way I'm going to haul it around on vacations, even with a Vagabond II power supply.
For now, I think the bottleneck in the quality of my images is my technique, not my equipment. I'm therefore going to force myself to learn my equipment better before adding equipment.