The original Destiny is totally free on Xbox right now, for some reason [Update]
Destiny is free, and has been for long enough that we're starting to think it's not just a classic console storefront cock up.
Vanilla Destiny is currently available for free on Xbox platforms.
Log into Xbox Live and you can grab the original version of the shared-worlds shooter on Xbox 360 or Xbox One at no cost.
[Update] Apparently you can't actually download the free version of Destiny (yet?), which is a bit of a blow to our "not a cock up" theory. We still haven't heard back from Activision on the matter.
This version does not include The Taken King, the September 2015 expansion which is now part of the base premium package of Destiny, but does seem to include two earlier, smaller DLC add-ons, The Dark Below and House of Wolves, which were folded into it when The Taken King launched.
The free version is listed as supporting in-app purchases, which is a reference to Silver, Destiny's microtransaction currency - also available in the premium version. Silver is used to purchase emotes and other cosmetic items, but it's not a huge part of the game - so it's more likely this is a gateway drug to Destiny: The Collection, which include's September's Rise of Iron, as opposed to the beginning of a full free-to-play conversion.
Xbox has accidentally made games free before, but this has been online for long enough now that we think it's legit. Neither Destiny publisher Activision nor Microsoft have made any sort of announcement about it, though we'll fire off some emails and see what comes back.
Destiny is not free on the PlayStation Store as of time of writing, but there's also no equivalent package available; the cheapest buy in is Destiny: The Taken King. I wouldn't be tremendously surprised to find a free version goes up on the PSN shortly.
If you enjoy this version of Destiny, you'll probably want to pick up The Taken King, which made dramatic improvements to the experience.