There's some true advantages to having this crude little devices - often found on commercial-size, or bulk-purchase containers of a limited number of aquarist's chemicals. They typically measure a specified amount - the ones I use produce 5 mL, or one teaspoon, per full stroke. In particular, I use these for two chemicals: Aqua Plus, which I go through rather aggressively, and Plant Gro, which, unfortunately, is not available with a pump of its own. I buy four of the largest bottles and empty them into a well-cleaned (NO SOAP) aqua plus bottle as a way to keep a handle on it.
Having a precise volume per stroke is very useful for these two chemicals in particular, where a certain amount, often quite a bit, is needed at once. Instead of goofing around with multiple measuring cups and spoons, you simply calculate the required dose, divide that into the number of necessary pumps, and inject directly into the incoming water in the water change or straight into the tank.
ThreadLots of plants don't like to be planted - Java Fern, to name the most common - and many others don't necessarily need to be. Getting these to grow on rocks or bogwood can be rewarding visually, and the easiest way to secure them until they have rooted is often to take a bit of uncoated thread (I prefer black) and affix it. A few weeks later, when the roots develop, the thread can be removed.
CO2 Injection RigYour average tank, even a planted tank, doesn't need CO2. Even heavily planted tanks often don't need it if fish are involved as well, since fish respire CO2 constantly, and under photosynthetic conditions, plants convert CO2 to O2 - at night, this cycle inverts.
I'm actually still getting used to the use of Carbon Dioxide injection. I've been playing with a little system for producing it, but in any of my tanks at the moment, it's more an extravagance than anything. A full post might be considered in the near future.
Electrical tape is a fantastically sticky, strong tape, with the advantage of being two things - jet black, and nigh-indestructible. I use it to hang printed backgrounds - a sold tape run down either edge, carefully, looks a professional seam and holds quite well, augmented with water to help secure the masking nicely. Since I don't have a guillotine cutter, it's far easier than trying to get nice cut edges on my backgrounds.
E-Tape is also useful in other ways - tacking a bit of line or hose, for example - so long as it's used externally. Adhesives are a bad thing inside the tank itself.
Straight, smooth edges are useful in a variety of ways, as are small, rounded corners. Smearing and smoothing aquarium sealant, smoothing out paper backgrounds, or invisibly propping up objects are all useful uses of tongue depressors and popsicle sticks.
In my history, they're also surprisingly useful as a lever, to either extract a part of something or open seals. This functionality became obsolete when I acquired a pair of stainless-steel tongs.
Cheesecloth, Panty Hose, and other Fabrics
My first use of fabrics in aquaria was as background material for a tank of an unorthodox side - a black piece of cotton affixed with background glue (something I no longer use) and edged with electrical tape did the job well enough, one supposes. I've also used it as skirting for stands without doors of their own, and once, briefly, as a screen to prevent fish from jumping.
More common uses now, are for more porous, or sheer fabrics. In my experience, panty hose material makes up the very best lining for a number of things, such as filter intakes in tanks planted with sand, or as an improvised filter media bag. The nylon is pretty much invincible.
I've also been known to filter new water through cheese-cloth as a sort of a net, especially when I'm working with sanded tanks, as a way to recover the sand from going into the bucket and being discarded with the rest of the water.