ah well, if thats the lowest % off you'd indeed be correct!

Flying Bundle 8

$1:
Desert Thunder (Steam)
Make it indie! (Desura)
Super Trench Attack (Desura)

>3$
Bionic Dues (Steam)
Duke Nukem (Steam)
Duke Nukem II (Steam)
Jack Orlando: Director's Cut (Steam)
Mystery Game (probably on Steam)

http://www.flyingbundle.com/

Terraria is 75% off (2.49€/$) at Steam for the first time this year, coinciding with the first content update to the game in a few months. As discounted as it's been or is likely to get, unless it's bundled someday. Deal lasts until Monday.

Edit: also, lol, just saw this "great" deal: with most of them discounted by 33%, all 175 of the Train Simulator 2014 DLC can be picked up for 1908€. Trai-zy. o_O

Forced on Humble Store 55% off/€6.29 for 7 more hours.

I still haven't figured out what kind of game Terraria is and whether I would enjoy it. Would it make sense to play it in single player mode? Does it have motivating objectives? Or is it more a game to fool around with, preferably with friends?

Yes, yes and yes ;)
Basically it's a 2D sandboxy world where you can craft and build stuff and fight many many enemies and bosses to find loot. And like most games, it's more fun with friends.
Or at least I assume so because I have no friends.

Edit: Oh BTW, I'm just answering the questions not recommending the game.

Well, I do have friends but at least regarding videogames I'm not much of a teamplayer; I actually enjoy some games more on my own than in the company of people pressing me on instead of letting me take my time and look at the scenery. ;)

Is there a story of sorts, an ending to reach, like in a metroidvania game, or is just about getting more and more powerful for its own sake, like an MMO?

Eh well my experience is from quite a while ago and back then there wasn't any story at all, I did read something about a quest giver in one of the latest updates. But that is also just for loot.
So I'd say it's even less of a story than a MMO, it's really just about combat, loot and crafting/building.
It really is very sandboxy.
Since the dev went on to make Starbound, it's a lot like that in case you've perhaps seen/read more about that game.

I think there are no objectives or goal based missions.

"Samphi", on the other hand, which has been released but is still under development I think, has or will have a story.

Basically agreed. I know I enjoy playing it single-player as well, though outside of multi-player it's probably a plus if you somewhat enjoy making your own gameplay goals. It does have motivating objectives like gradually stronger bosses (though for the most part you can tackle them in any order you please), discovering hidden areas (floating isles, pyramids etc.) with unique loot, but it's your choice whether to pursue them or just make ones of your own, like constructing complex structures with automated farming, defense systems etc. Aside from the creative aspects, combat is IMHO entertaining, with solid mechanics and huge potential for customization. All in all, it's a game that's hard to summarize because it's very rich with variety. :)

Where are you seeing the timers at? All I see is that it states: "New deals daily at 10 AM Pacific"

It does have a story, but it's loose... there are "recruitable" NPCs (they'll appear and stay in housing built for them) which give you hints and directions on what to do. And, basically there is an "endboss", but if you know where to go you can head straight for it - though without decent tactics and equipment I don't think he can be beat - which after he's defeated the game enters 'hardmode', with new biomes, material and tougher enemies appear, elements of "good and evil" start spreading... but again, whether to deal with or ignore the new environment is mostly your own choice (except for the fact that almost everywhere becomes more dangerous to explore, heh).

The dev did NOT go on to make Starbound (which without modding still feels very shallow to me compared to Terraria, though admittedly it's still early in development), but Starbound's head dev was the pixel artist on Terraria for a time, before starting his own team. Terraria's dev is starting work on Terraria 2 sometime this year.

There are a lot of "Terraria-likes" in some stage of development; the best one I've played so far is Darkout, which while it is very recognizably unfinished, already has a stronger implemented story than e.g. Terraria and Starbound.

Eh whatever, dude worked on Terraria and now Starbound which are extremely similar games.
Was the head dev the one that had a hissy fit and quit development on Terraria for a year or two?

Yeah, there are many similarities between the games, just tired of repeatedly seeing the games being referred to as having the same creator. Terraria ceased development for about a year, yeah, but IIRC it was a case of burnout rather than a "hissy fit" (until last year it was almost a one-man project, there's a larger team on it now though).

See attachment; you don't see that countdown in the bottom left corner when you load the Humble store page?