
Generations ago, humans fled to the cosmic anomaly known as Grass. But before humanity arrived, another species had already claimed Grass for its own. It too had developed a culture...... Now a deadly plague is spreading across the stars, leaving no planet untouched, save for Grass. But the secret of the planet's immunity hides a truth so shattering it could mean the end of life itself.
In particular, SF plots often mix in significant fantasy themes to the point that they are sometimes a majority of the book.
Banks' Inversions is a Culture novel (SF) that intentionally reads as fantasy if you don't know the Culture setting, while leaving the reader to infer the SF setting if you do.
"Grass" by Sheri Tepper, or "Left Hand of Darkness" by Ursula Le Guin are other examples. Both happen on other planets, but while both have SF settings, most of the stories fits better into fantasy tropes.
Vernor Vinge's A Fire Upon the Deep is very explicit SF at "top level", but a significant part of the plot is closer to fantasy, happening on a planet in a pseudo-medieval setting.
Others like Stanisław Lem's The Cyberiad, while more explicit SF, also intentionally mix the two - being written as fantasy in a mock-medieval inspired proto-feudal society, where the characters engage in typical fantasy-inspired quests, with dragons, princes and princesses, with medieval weaponry, but with most characters being robots and with access to space travel...
There's a lot of overlap where authors toys with the distinctions, or outright mocks them.
Anyway, one title I didn't see mentioned was Grass by Sheri Tepper, https://en.wikipedia.org/wiki/Grass_(novel).
The Long Tomorrow by Leigh Brackett
The Doomsday Book by Connie Willis
Among Others by Jo Walton
Feed by Mira Grant
The Hunger Games!
The Hundred Thousand Kingdoms by N.K. Jemisin
Grass by Sheri S Tepper
Gate of Ivrael and Downbelow Station by CJ Cherryh
And the fabulous space opera:
Ancillary Justice by Ann Leckie
Here is a link to the "first" in the series: http://en.wikipedia.org/wiki/Consider_Phlebas
Basically to give you a flavour, imagine a utopian society where energy is basically free and run by super intelligent benevolent AIs. So basically the citizens have nothing to do but live a completely hedonistic life.
Whenever they encounter "primitive" societies, they try to make them more human and civilized. Basically the opposite of the "prime directive". The organization responsible for that is called Contact. Unfortunately, in order to make these societies more humane they sometimes have to resort to some dirty tricks (like assassinating evil dictators, instigating civil wars, etc etc). The organization responsible for that is called "Special Circumstances".
Another author I find great is Sheri Tepper. http://en.wikipedia.org/wiki/Sheri_S._Tepper
SciFi with a feminist bent. Examples are Grass / and the Margarets.
If you want something more acid, anything by Victor Pelevin. Buddha's little finger is brilliant.