For when actually existing banknotes just aren't enough... Most of these I've found randomly browsing eBay for banknotes to add to my collection. I admit I initially rolled my eyes at such things, but some of them are just interesting, amusing, great artwork, or some combination thereof. I have to concede the point that many of them look better than some real banknotes I've collected (or wanted to collect). Some of them could have been real ones, honestly. I don't actively seek them out often, but sometimes one will catch my eye and I get the "I got to have that" impulse. There's not a "fantasy" or "novelty" category on the site I use to catalog my collection, so I've decided to start labeling my fantasy notes as "exonumia" which arguably they are. (I suppose they're the paper equivalent of medals.) Some of them don't even pretend to be from actually existing countries. I was debating whether the ones that do pretend to be banknotes of a real country should still be categorized under that country, but "exonumia" seemed to be the best way to separate them from real banknotes that are actually legal tender in that country (or at least used to be at some point in the past).
A few of my favorite examples from my collection: What modern Algerian notes might look like if they still used the French style of the 1940's. Fantasy note issued to commemorate Slovenia adopting the Euro. If the Vikings stayed around along enough to create paper money. Um, dinosaurs, because why not?
I admit to being tempted by these fantasies and also the odd test note here or there. In general though I resist as I know I would completely slide down a slippery slope if I took the step. I do like that Algeria one though...
It's kind of the same thing with coins; I have little to no interest in coins that are in the shape of a donut or the Batman logo and such, but sometimes one will just catch my eye in a moment of weakness and I'll go ahead and grab it. I saw a dragon coin that came in a dragon egg once and I went ahead and bought it before my wallet could react in disgust and say no lol. There's so many real notes I don't have yet and would like to have, so I don't do this often with banknotes, but some I just can't pass up. Especially the ones that are trying to look as if they could be real banknotes, but a little better. Like that "Algerian" one; that could easily have been a real one. My comment to the person I bought it from was that it should have been. In the end, it's your money; buy what you like, don't buy what you don't like, and don't feel a need to justify your decision to strangers either way. I do this for fun, but what constitutes "fun" for me changes from time to time. P.S. Not trying to tempt you, but if you liked that Algerian one, you'd probably like these ones too: There's probably not nearly enough commerce amongst the handful of scientists in the Kerguelen Islands to need their own banknotes, and what few monetary transactions that take place there are probably just done in euros, but love the idea of these anyway.
I like the Algerian one best, though. Here's a set I bought some time back, just because. I could easily get tangled up in all these.
+1 I like @Dave M's musical set too but I can't justify buying any fantasy notes when I consistently go over my budget on real World notes issued. I really like to patronize the arts (& its super tempting) but I just have to draw a line in the sand somewhere. Otherwise, it would also be specimens, hyperinflation, notgeld, you name it! Nevertheless, nice adds here @Troodon!