Maybe one for the Wiki, feel free to chip in.
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Back nuts & tap connectors can be really difficult to remove due to age, corrosion & tight spaces. Several different tools have been developed to make this job easier.
A basic back nut wrench, often called a 'crowfoot', can be purchased for a few pounds;
Adjustable basin wrenches are the next step up. Relying on a spring loaded serrated jaw, they will automatically fit any size of tap connector or back nut. Models with telescopic handles are more versatile because the length can be adjusted so that the tommy bar is easier to turn.
The disadvantage is that the head is sometimes too large to get into the space available and that the jaw often slips off of the nut.
More specialised tools like the Armeg Jaw Dropper solve many of these problems - but at a price.
They are both double ended and have two 'jaws' at each end, with a sliding tommy bar in between. The outer 'jaws' are hexagonal to grip back nuts/tap connectors, the inner 'jaws' are simply curved & can be slid out of the way.
In use, the appropriate end is selected. For a tap connector the curved sliding jaw is slid up flush with the hex jaw & locked into place. The wrench is then slid up the pipe and almost automatically locates itself on the nut. You can often do this without being able to see the tap connector and it won't slip off.
For back nuts, the curved jaw is slid back out of the way to give the tool a smaller profile.
The Jaw Dropper works extremely well in almost all situations. The only downside is that the tool isn't long enough to always allow the tommy bar to be rotated fully.
A recent addition is the Todays Tools "Professional 1/2"- 3/4" Tap Back Nut and 15-22mm Compression Fitting Set" which an be seen here;
A bit more reasonable in price, I paid around £27 for mine.
You get two split ring spanner heads & two box spanners, all of which fit a 3/8" drive ratchet with extension bars.
You can't use the box spanners to undo/tighten back nuts without disconnecting the pipework and you have to make sure the split ring spanners don't slip off the tap connector nuts, but the ratchet & extension bars are a real boon in tight spaces with badly corroded fittings.
Which is better? It depends on the job. I'm glad I have both.
The removal of monoblock mixer taps requires a set of box spanners, but I have just noticed a set of sockets on the next page in the Todays Tools catalogue which are going on order.
HTH