Maybe The Witcher, which was developed by CD Projekt, owner of GOG.

Nasz Wiedzmin jeszcze nie jest taki stary :-D

Maybe fully working Gorky 17.

Ale wiesz, mamy spodziewać się "unexcelled". :)

@Kola256-
On jest najlepszy....ale ja chce Garreta :D
(forum znowu wyprawia czarne magie...nie moge odpowiedziec do Twojego postu...)

All I want are the WIZARDRY and ULTIMA series'. I've been waiting for those ever since I joined the GOG beta 452 days ago... just been hoping for those for a loooong time. :)

What about the Ultima Underworld games?? I loved those back in the day...

Let it be EA. That would be a BIG news. Lot of old good games from their catalogue would empty my wallet in a second.

EA would be awesome, so many games...
Origin and Bullfrog would be very nice (both owned by EA, and would give us Ultima, Wing Commander, Dungeon Keeper etc.)
[edit] not to mention Westwood Studios!

I wouldn't read to much into the background if I were you. It's got a slightly horizon-like quality to it, but the curve is rather subtle and there's no real texture on the lower surface. The star-field image above is probably just for dramatic effect (better then some more block colour or gradient), it gives the whole image a larger scale. It does, however, suggest something larger then their average announcements. Oddly though there's no corresponding features or news, just the countdown so relying mostly on the mystery of it all to keep us hooked and watching (although I'm pretty sure most of us would still be here anyway).
It seems the only real clue is in the text, the phrase is obviously a variation on the common "expect the unexpected", so of every thing it's only the word "unexcelled" that could really provide any real clues. It probably doesn't refer directly to a title or name as the whole word is in lower case, the typeface isn't unusual to the site and the sentence is normal in it's punctuation ( the full-stop's a bit odd though, you would expect a '...' , though as it doesn't seem to be a title, as such, I think it's probably nothing). I never see the word "unexcelled" used before so I have to think it's use was a clue of some kind.
Ignoring the use of this unusual word for the sake of its uncommonness, I'd have to conclude from it's meaning that the 'event' is a release of a game, one of the all-time classics. Of which I'm afraid my knowledge on these is a bit sparse, although I say it was some thing from the top of the wish list, possibly Planescape: Torment; I've seen this has been republished recently after a long absence, this is my best guess (if I'm completely wrong about the background image's significance though I'd also bet on the System Shock 2). Things that I believe are fairly certain are; It has to be something old (Good OLD Games, for one thing), time has to have passed for other titles to not have "excelled" it and it must be something really, really, really good to be worthy of this mysterious, and unexplained, countdown.
There is the possibility of a new, and major, developer, but it's still only likely to be a single or small number of new games. If you release every thing at once what are you going to do next week? I also think a new site or feature is unlikely as, again, "unexcelled" suggests it's been around a while.
The date could be significant; the only thing (vaguely) relevant thing I could find was 'Data Privacy Day', which seems pretty unlikely.
What we can all be certain of, though, is that the site is going to be very busy on the 28th!
(Image continues GOG's excellent graphic design record, love the site's look.)

Personally, I wouldn't hope for the countdown to concern games being released here that can already be downloaded digitally. To me, if the games can be found elsewhere digitally, it doesn't warrant a countdown or anything.
Like others have said, my money would be on a new publisher deal and a pretty big one.
Edit: While I'm not hopeful for it, Planescape would indeed be nice. It's pretty different from other roleplaying games in terms of story/setting etc. and the writing is absolutely terrific. I say that as a writer myself. I'm not sure I'd buy it though as I bought it a while back for �5 - �10, the old white label version. I think it still works, but not a 100% sure.

That would be huge! Not sure the gog.com servers would be able to handle the surge of interest.

I hear ya man, I've been wanting the Wizardry and Ultima series to be released on GOG myself. I wouldn't count on Ultima though as it's owned by EA, but Wizardry is definitely possible as GOG already has other games from the same developer and publisher Sir Tech and there was an old Wizardry collection published by Interplay who has released alot of other games on GOG.

Amazon.co.uk has it for 11.17 Pounds. That's roughly twice our higher price-point... As much as I'd love to get my hands on P:T - it's unlikely.
Don't forget that GOG is THE place for extremist anti-DRMers. As such - there's certainly a group of people (including me) who would appreciate such titles, simply because they refuse to buy them elsewhere.
Blasphemy ;P !