3) Needle/Metering Valve
The next piece of equipment that is essential is the needle/metering valve.
A needle valve is a piece of equipment that takes the delivery pressure of the regulator and further drops the pressure down to the very fine flow rate that we require for aquarium purposes (i.e. we often refer to our flow rates as "bubbles per second"). A metering valve is the "high end" needle valve.
Needle valves work by restricting the flow of gas via a small needle (hence the name) that can be opened/closed via a screw/caliper handle. In general, higher quality needle valves/metering valves will have allow finer control by having more threads. This means that it takes more turns of the handle to change the flow of CO2, meaning you get finer resolution (i.e. if you turn a needle valve 1 turn and get an increase from 1 bubble per second to 10 bubbles per second, you would have a hard time adjusting your flow. However, if you turn another needle valve 1 turn and only get an increase from 1 bubble to 2 bubbles per second, you can achieve much finer control).
A good quality needle/metering valve is essential. This is definitely one piece of equipment you do not want to be stingy on.
Here are some brands that I recommend:
Fabco (particularly the NV55)*
Ideal (particularly the 52-1-11)**
Swagelok (many various models available)
Parker (also many various models available)
For those that are more technically inclined, have a look at the thread over at the Barr Report (linked above), as it discusses the finer points of a quality needle/metering valve (i.e. best Cv to look for, etc)
One brand of needle valve that I would strongly advise against is the Clippard needle valve (Part #: MNV-4K2) . While it is quite cheap (perhaps $18, if ordered online), many users have lamented that the quality of this particular needle valve leaves much to be desired. A common problem with this needle valve is that it "floats." This means that while you set the CO2 flow rate to a particular setting one day, the next day (or perhaps within a few hours!), the CO2 flow rate will change noticeably, requiring more fiddling on your part. This means that while you set your CO2 to an "optimal" flow rate one day, the flow might stop the next day, or it might be so high that it will gas all your fish to death. Definitely, this is something you want to avoid, so do not be stingy on a quality needle valve.
*Note 1: The Fabco NV55 contains #10/32 port fittings. These are not your standard fittings and adapters cannot be purchased at the hardware store. The setup I would recommend is to have #10/32 to hose barb fittings and not trying to find #10/32 to (say) 1/8" NPT adapters. This is because attempting to attach the Fabco NV55 to the regulator is not a good idea. The Fabco NV55 is quite a heavy needle valve, and the #10/32 fittings are quite small and fragile, so a slight bump may cause the fitting to break. With the hose barb adapters, you can run this needle valve in-line.
**Note 2: This particular Ideal metering valve has 1/8" female NPT ports on both ends. Other models exist, and I can also forward you the PDF/website with the particular details if you require/PM me.