Firstly, all of the opinions given are personal !!! Over the years I have bought, tried, & disposed of numerous items which initially were bought with the thought "Geez, that's brilliant. What a difference that will make, I must get one, or two …”. However, in recent times I have become more skeptical - although I still make mistakes.
Likes, & must haves...
The ArcaSwiss dovetail systems, as sold by Acratech, Really Right Stuff, Wimberley & others (inc. ArcaSwiss funnily), has been one of the best additions to my set up.
Bubble levels, both on camera & tripod/head mounted as made by Hama, Acratech & others are must haves for leveling your gear. Other must haves on this front are Acratech's leveling base & Gitzo's G1321 leveling base.
The GT2931 basalt tripods from Gitzo are almost as light as their Carbon 6X counterparts, but are considerably cheaper. I have been using 2 since late 2006 & they are superb, as is the Acratech Long Lens head of which I seem to have been one of the first people to have one, receiving it in very early September 2007.
LowePro's DryZone 200 backpack, which is fully waterproof & can carry an EOS 1D body, 300/2.8, 70-200/4, 105 macro, 24-70/2.8, 17-40/4, 580EX & some other small bits & pieces. If you live in the tropics, or Scotland - or other places equally as wet - &/or jump around near salt or fresh water, it's a super bag to have. You can easily carry a 15" MacBookPro in the outer (non-waterproof) pocket too.
Apple computers - see Apple V's PC.
Gepe Card Safe flash card cases. Small, light, watertight & able to hold 4 CF or SD cards, they are relatively inexpensive, tough & well designed. Avoid card wallets using Velcro closures, they can be noisy just at the wrong time.
Less glamorous, but equally necessary whilst living & working in hot & humid climates are humidity control cabinets & hermetically sealed cases. The main reason for my change from Minolta to Canon in 2004 was several factors contributing to one outcome – the gear was either dead or dying. A mixture of dust, humidity & too many miles of travel had caused the camera circuits, & their optics, to become degraded past an acceptable level. Several functions & display no longer worked. Images through the viewfinder were getting duller & less sharp. Therefore, when living, humidity protection has become a prerequisite when it comes to gear - hence the Eureka dry cabinets, Pelican cases, & the ample sachets of Silica gel distributed around the various (hermetically sealed) storage units & cases.
Dislikes, & mistakes include...
Tripods advertised as being fully adjustable in all directions, to any angle, & waterproof too. I bought one – had it for nearly 7 years, used it very occasionally – & cursed it every time I used it. For me, it was too big & too heavy for traveling (unless you have a horse in tow). Setting it up &/or moving it around was like trying to help a disorientated, one-eyed, paraplegic octopus across a busy, ice covered road whilst wearing roller blades. Certainly, this purchase was not my wisest decision. The only plus as I remember was I got a super price for it on e-bay !!!
Too many purchases of camera bags to mention - including many from both Tamrac & LowePro. Shoulder bags are okay for me for working from a car or fixed location, but to carry 18 to 20 kg's of gear over one shoulder for long periods of time is a killer.
Backpacks have been another failing of mine. Although they spread the weight over both shoulders & hips, getting one that can actually accommodate all the gear I want & a laptop has proved elusive. Currently I have just bought a LowePro Vertex 300 - time will tell if it is the one for me.
Other items found to be less useful than envisaged include 50 mm Macro lenses (too short for Macro work), tripods that whose working height is considerably less than your height (too much bending & back pain), & numerous other things that we'll leave for another day.
If you do have something you need to move on, try an ad in e-bay, it's easy, relatively cheap, & very effective. I've sold numerous mistakes via them.