Indiegamestand.com sometimes gives away GOG keys with their deals. Also Groupees.com did it sometiems in the past with some of their bundles (but it seems they don't do it anymore).

Besides that, I don't think there are any stores that sell GOG keys on a regular base.

I think there aren't. As far as I know the only exceptions so far have been the ability to redeem any Witcher and Witcher 2 keys on GOG + the partnership with Curse recently which allowed people to get a "free" GOG game by signing up for a premium membership there.

Edit: Ninja proven wrong. It clearly isn't my day.

And also IndieGala. ;)

Currently on IGS:
Ring Runner: Flight of the Sages

The Real Texas

Waking Mars

SpaceChem

edit: actually scratch that, the games are listed as a platform GOG.com but there is no mention of GOG keys on the individual pages.

The problem is that GOG is not necessarily a 'platform' as much as a DRM-free store, so they haven't looked into advertising themselves as one and publishers haven't felt the demand to include key options for it. Bundles get GOG games included when the developers/publishers provide them, which is only because they're already selling them here.

It's also up to the publishers to decide how many platforms they want to provide keys for. Amazon used to only provide retail copies of a title until people asked them to ask publishers for Steam keys. GOG users would likely have to do something similar, while GOG itself would have to become a platform and not just a site/store/ideal.

Going to a store to buy a game code that unlocks on another store...a little odd. ^^

Why would any GOG user want to request that?

Precisely. The ones asking for Steam keys for amazon/greenman games proved to be substantial. GOG users aren't vocal about not buying a game if it's not on GOG when on sale on another store for retail/steam/origin.

I wouldn't mind resellers of GOG keys. It typically means more sales more often. I'm not aware of any downside...

Ring Runner, Real Texas, and The Cat Lady were three games featured on Indie Game Stand spotlight deal that actually game with GOG keys.

Zafehouse Diaries GOG key was available from Groupees (Bundle of the Damned 2).

Indiegala featured a Creatures Exodus GOG key in the Limited bundle recently.

I've never understood why one would want to have a game from a particular store, but not just buy it there. Seems very strange to me.

In Gog's case, I'm not sure how much sense it would make for them to do that on any sort of regular basis.

If that retailer has its own client, like GMG, then I'd want Steam keys as well, just so I don't have to install so many crappy clients. For me, it doesn't matter with DRM-free stuff, like GOG, Desura, Shiny Loot, GamersGate, Humble Bundle, et al.

Tremor Games give some GOG keys as art of its rewards, thought most of the games of the site are Steam.

And you know what?. The reason people buy Steam keys from places like GMG,GG,GGG,Amazon etc are because of the cheaper prices.

When I bought Defender's Quest from Level Up Labs, I asked about a GOG key and they said: "we do in fact give out GOG keys, but GOG lets us do this only if we don't openly advertise it." Kind of a strange policy, but it does explain why it's hard to find GOG keys elsewhere.