Some important tools for modern photography are not cameras or lenses, as strange as that may sound. Those tools range from speedlights (flashes), monopods, tripods, wired and wireless shutter releases, batteries, memory cards, lens and camera cleaning tools, post processing software and more. As useful as all these tools are, they have strengths and weaknesses, just like the cameras and lenses we own.
To start I'll discuss the advantages of many of the tools I listed, and then dive into some of the issues and disadvantages of them. Since a photographer can get away with not using many of these tools, the weaknesses of the listed tools can turn many users away from them, but the strengths of tools like flashes and tripods can often outweigh the disadvantages.
Flashes/Speedlights: Tools of light, but often bulky
Flashes/Speedlights are powerful tools with more uses than I have found for them yet. The ability to give light to a pitch black room, or to create fill light to eliminate shadows makes them a necessary evil in many respects. Then there is the ability to use colour gels on a flash head to change the colour and mood of an image, something you could not otherwise do. Flashes have a big advantage of studio lighting in that they are relatively light weight and easy to transport, unless you have a more than a few of them. In the modern age one of the greatest changes in flash technology is wireless activation, which allows a photographer to have multiple speedlights, and being able to control them all from a master unit, either mounted on the camera or via the cameras built in flash.
The weakness of speedlights is that you have to carry them around with you, just another thing to weigh you down. Now, if you can drive to the location you shoot at, having more than a few flashes can be great, but once you start working with five or six units, then it can be a burden if you are walking a reasonable distance. Then of course there is the issue of needed batteries, a defuser for your units, and other flash related accessories. From my point of view the value of having one or two flashes far outweighs the negative aspects of having them.
Tripods and Monopods: Stability and transportation
Tripods and monopods are a tool that can change the way we take photos, by adding stability to our equipment. Just try taking a long evening or night exposure without a tripod, the results wont be very pleasing in most situations, unless you are going for a abstract image. I have heard some people say that tripods are no longer needed thanks to IS/VR, but I have not found that to be true at all, then again I don't have a D3s that I can use at ISO 6400 or higher. A tripod or monopod can give you a great deal of needed stability when you are working with telephoto lenses as well, and considering how hard it can be to handhold some higher end glass, it can mean the difference between getting the shot you want or not. Tripods can also be useful for taking images that need to be stitched together, such as panoramas. There are methods of making panoramas without a tripod, but I doubt the results are as good.
The only real downside that I have found for using a tripod is having to carry it around, and the cost of getting a good one. There are tripods in almost every imaginable price range, from $50 - $1000 or more, but from what I've seen most of the ones under $100 are not even worth looking at, unless you are using them with a compact point and shoot, or a entry level camera with kit lenses. If you can afford a carbon fiber tripod then some of the weight issues can be dissipated, but you still need to take it with you. Another option if weight and cost are and issue is to get a monopod, which will often give a great deal of stability, and you can use it as a walking stick, which is a bonus!
Other Accessories: Cable/Wireless Shutter releases, batteries, memory cards
Some of the often overlooked, but valuable accessories to our camera gear are tools like cable releases and memory cards. It is kind of funny that I talk about cable releases, because one night when I went out shooting I forget my release, thankfully my camera has mirror lockup, so I was able to get away with it, but it would have been nice to have! The nice thing about cable releases is the ability to reduce the chances of causing camera shake on long exposures, then again, you can end up doing what I did and forget to bring the remote along. Camera, and flash batteries are important tools to keep us going, without them we wouldn't be taking photos, not digitally anyway! The disadvantage of having to use batteries, specifically of many DSLR camera is that most do not use standard rechargeable batteries, which means you have to buy expensive branded batteries. There are cheap alternatives, but keep ind mind that any damage done by a third party battery, could void your manufactures warranty. For me, it just is not worth the risk. The type of memory card may not sound important, but consider this, without them you've got nothing to record your images on, since when you turn your camera off the buffer clears and any images taken will be lost.
For the average snap shooter any kind of memory card will work just fine, but if you are working with higher end cameras, like the Nikon D300's or Canon 7D, then you want to have memory cards that can keep up with the camera. After all, what is the point of having cameras that can shoot at 8FPS, if your memory card cannot record the images fast enough to take advantage of that? For some that will not matter, but if you are into sports photography, or shoot wildlife you might want to consider investing in some high quality, fast memory cards. The downside to high end memory cards is price, while I could get a cheap 4GB CF card for $18, a higher end 4GB card could cost $40, $60. I've never had a memory card fail on me, so brand is not important, but all my current CF cards are from Sandisk.
Another accessory you might want to have with you is a cleaning kit. I always have my blower in my bag, just in case I get some pesky dust on the front my lens, or mirror after a lens change. I find the sensor cleaning technology of modern DSLRs works well, and I haven't had to do any sensor cleaning since I sold my D80 last fall, which is a surprising considering how often I change lenses outdoors, even in windy conditions. A lens pen can also be useful when you are trying to deal with pesky dust that your blower cannot remove. Of course the downside of such cleaning equipment is that, if used improperly, you could damage your lenses and cameras. If you are too scared to clean your equipment yourself, you could get someone with more experience to show you the right way to use it.
Well, I think that just about raps up the series on strengths and weakness, for now anyway.