[nas]

You can make due with only a couple of good knots. Very short list: the reef knot (square knot, avoid the granny version) and the alpine butterfly (don't use reef knot for joining two ropes, i.e. a "bend"). The alpine butterfly is more useful to know than the bowline, IMHO.

If you want to expand your list a little, here are some additional useful ones: double fisherman's, adjustable grip hitch, sheet bend, trucker's hitch.

Edit: I suppose this is more useful with a little additional commentary. The reef knot is so common that you should know it and know how to avoid the granny knot and also when not to use it (e.g. as a bend). You can use the alpine butterfly as a bend and also for quite few other things. It is more versatile than the bowline (e.g. if you need a loop that doesn't slip) and works fine as a bend (very smiilar to the Zeppelin bend).

[at_a_remove]

Right, but this isn't a what question, it's a when to do what question. I might know a knot, but I don't know when to use that knot and when it ought to be avoided. That's what I am getting at.

Even under the "bend" page in Wikipedia, sometimes the knots are merely described.

[marai2]

Be the change you want to see in the world ;-)

I think a flow chart of when to use a knot is a great idea! Please make it!

[at_a_remove]

I would first have to know what I was doing. Which I do not.

I might be able to make the flowchart if I understood knots; I might be able to understand knots if I had a flowchart. If we had ham, we could make ham and eggs, if we had eggs.

[eternityforest]

If it's a safety critical situation, leave it to the experts/become an expert/try to make it so that the failure of your knot won't kill anyone/etc.

For non-critical use(In my limited experience, some of my choices are as much about ease of tying as actually picking the right one, some of these are things I've only used once or twice or never used for anything important):

If you want to tie two ropes together, use a sheet bend. or a double for more security, or the slipped version to be able to undo it. Add a stoper knot for even more security.

If you want to make a loop in the middle of a rope, alpine butterfly.

If you want to adjust tension, tautline hitch.

If you are tying thin spectra cord, or making a permanent attachment between thin cord and something else, try a uni-knot. Note that it's not actually considered the standard for tying to carabiners, but I use it sometimes(For non climbing purposes).

If you are tying a bundle together, try gliepnir or a standard shoelace knot, or a clove hitch.

If you are tying a rope to a fixed pole, try a clove hitch. It's not perfectly secure. Maybe try gliepnir too. Honestly bundles and hitches are the two uses I'm least sure of. There are apparently better things than the clove hitch, but it's easy and good enough probably.

If you want to tie something you can undo really, really fast, use a highwayman's hitch.

If you want to tie something someone else might need to untie, shoelace knot which is a doubly slipped square knot, it advertises visually that it's meant to untie.

Might want to just use a regular overhand though, the Ashley stopper is hard to undo.

If you want to cause a real nightmare for anyone untying something, or remove unwanted fingers by circulation loss, constrictor knot.

If you want to make the end of a rope not go through a hole, Ashley stopper. You can also use this, or just a simple overhand, to keep another knot from slipping.

If you want to do something involving climbing, do not listen to me, I know nothing about it.