Kagi is a service that acts as kind of a
institutional middleman between
shareware authors and
shareware users. The thinking behind it goes that shareware authors should only have to write their software; it makes no sense to have to completely reinvent the wheels of
payment and
registration and
crippleware for every single shareware program out there-- which you'll notice people seem to do. Logically, if you have some
common task which a great many different computer applications will be performing, it makes more sense to farm that out to some kind of
standard library than to have everyone implement the task themselves.
So, Kagi performs that one task for the author. They provide a mechanism for funnelling money to the author, set up serial numbers and everything for the author (which is good because if the author wrote their own, it would probably wind up cracked anyway..), give the author some code they can insert into their program to check the serials, handle registration, and put up crippleware; and then give the author a program to distribute that will help the user find some way to pay for the program, and then help the user find a way to get the serial number for the program back from kagi. (Generally you wind up giving it personal information and credit card number, which it encrypts and spits out as a block of garbage you can e-mail to kagi.)
I am not aware of Kagi having any presence in the windows world, but in the mac world they have been nearly omnipresent; few are the mac users who have not seen that ubiquitous little app named "Register" with the icon of a $ superimposed on a blue diamond; few are the mac users who have not spent at least an hour or so within their life waiting for the 10-second-delay "REGISTER THIS PROGRAM WITH KAGI!" annoyware box that some of Kagi's clients choose to use.
At some point recently kagi has tried to abstract their focus away from just the registration stuff and set themselves as a full-fledged "online store" dealing in intellectual property of all kinds, including music. I have not, however, personally used this part of it.
Kagi is one of those very rare instances where literally everyone wins; the user gets security and flexibility of payment, the author has a huge bunch of really irritating stuff related to handling the money coming in that they don't have to bother with, and kagi gets a percentage.
http://kagi.com/
"Kagi" is Japanese for "key", hence the name. The website also says that "Kagi" is lithuanian slang meaning something vaguely along the lines of "WTF?", but that that's just a coincidence.