A number of software products these days are DualLicensed. In practice, the term refers to products written by commercial entities and provided under a Free license for private use, but also sold for commercial use under a commercial license. The canonical example for the Free license in use is the GPL and the canonical example for a successful DualLicensed product would be MySQL.
To a layman, the legal implications of this constellation may be confusing. Once downloaded under the GPL, any redistribution of the product to anyone (including businesses) must be under the GPL. However, if you obtained your copy under a commercial license, you are bound to the terms of that license and may not freely redistribute that copy. Of course, the company may (and, obviously in all likelihood, will) refuse to directly support users of a copy obtained under GPL.
Thus, the company and its customers get the best of both worlds: everyone can use the product and create derivative works, but there is also a commercial entity behind it which will offer contracted support, so business owners and PHBs can sleep better at night.
DansGuardian used to employ such a method, but now is only licensed under the GPL. It is sold for commercial use from the author's website, but that doesn't prevent you from acquiring it from another source. --JohnMcPherson
Right, and that's exactly "DualLicensed" means. a) You get it under GPL: you may redistribute it and must do so under GPL. b) You buy it: you may not redistribute it. c) You get it from a third party: because of a)/b), this is only possible if the third party obtained it under GPL, so you get it under GPL, even if you'd have to buy it if you were to get it from the author. This is why I wrote "the legal implications may be confusing". Exactly the same applies to MySQL -- you can acquire MySQL from a third party under GPL, even for commercial use, except MySQL AB won't support you then. --
AristotlePagaltzis
Not true, according to 
http://dansguardian.org/?page=copyright2 - you download it, and then it's GPL. The page seems to be heavy handed wording to try and make you think you can't do certain things, but once you've obtained it, I see no reason you can't take it and do whatever you want with it, as you are allowed to redistribute it under the terms of the GPL. -- CraigBox
Ah, sorry. Duh. I see:
So it's actually not DualLicensed. Well, I guess the way he set all that up is stupid.. but whatever, eh? --AristotlePagaltzis
A pioneer of the general sense of dual licensing was Perl, which is distributed under the terms of the ArtisticLicense? as well as the GPL. However, since it is Free under both licenses, it is not the canonical example and not what people generally refer to as DualLicensed. The difference is that GPL is a CopyLeft license, while the ArtisticLicense? is not. Perl's situation is unique because no circumstances force you to explicitly commit to either license, so you can redistribute the package under both licenses yourself.
3 pages link to DualLicensed:
