The German company Dallmyer has come out with a Nespresso compatible capsule, which they call "Capsa".
It's not available everywhere, but can be ordered through the German Amazon site Amazon.de.
I now prefer this to the original capsules as they are easier to store (they come in small cubical boxes instead of sleeves), and are slightly cheaper (about .30 euro cents per). Quality is very good; I would say equal to the original.
I do love my Nespresso machine on board. The running costs are fairly high but the machine itself was
cheap, and it is much more compact than normal espresso machines. I probably wouldn't use it on land, where you have more space to faff around with grinding beans and
cleaning out the baskets of a normal espresso machine, but on board where you have neither time nor space for that, it's really perfect. The only thing which might be better would be a really tiny bean-to-cup machines, which AFAIK, doesn't exist, and which anyway would not pay for itself except for people who live on board year round and/or drink tons and tons of espresso.
Bravo to Dallmyer for finally breaking the Nespresso monopoly. Since the patent on the capsule expired, there have appeared many compatible capsules, but all of the ones I have tried are dramatically worse in quality than the orginal, without being dramatically cheaper.
I used to think that Nespresso capsules were really expensive, but I think now that it's probably not more than double, or maybe 2.5x the cost of using really good beans (which Nespresso capsules use), and the economy of labor, time, and space is enormous.