I was hesitant to chime in with my unwanted tuppence, given that I have some strong feelings about personal expression and the freedom thereof. Thankfully, Unice captured a great deal of what I wanted to say far more eloquently than I could have put it.
I have to emphasize avoiding 'cultural hypocrisy' when it comes to swears/curses/expletives.
Mathtans covered a very important point about professionalism and how a person's day job can really dampen freedom of expression creatively.
Everything everyone has said so far is very good food-for-thought about this whole thing. The only thing that piqued my interest is the shifty sentiment I see that swearing/cursing is somehow morally inferior, less sophisticated, or makes a character an asshole. Don't agree with that. Quite the opposite in fact. Prudes tend to be assholes far more often than uncouth bastards are. That's just my personal experience for what little it's worth.
I agree with Bluepencil about the word 'fuck.' The word alone or used in a modernized context can be jarring. I have to waffle on this though, because it is a versatile word, and sometimes you just have to drop the f-bombs. There can be room in fantasy for reasonably appropriate f-bombs.
Lots of stuff has been mentioned so far, so I'll try to mention some in-context swears that I've used. If you want to criticize me for them, feel free.
I try to have my characters swear very sparingly, but I will when it's absolutely relevant to the situation or the character. I will try to put the swear in my fantasy world as much as possible. Most of the swears do come across as religious, given it is a fantasy society.
The most jarring use of a swear I have thus far is in a flashback I just revised and uploaded. In a way I wanted to hearken back to the Queen Xenomorph scene from Aliens. One of the characters finally comes into her own with her telekinetic powers, and pushes her individuality against her mother. It's an act of rebellion along with wanting to protect her family against a monster. Right before her offensive act, she blurts out, "Fuck off! You undead bitch!" I'm still debating changing it, but it fits for now.
Another character is a bit of an upstart that has a lot of heavy resentment towards a main character. She's headstrong, arrogant, and doesn't like the main character having abilities that she doesn't have. When the main character uses her abilities to sabotage her (causing her to trip in a run) her pride is hurt. She blurts out "Dhaulmyrr (sun god) fuck you with his ever-burning flames!"
I have some rough-and-tumble barbarian-like characters who are warriors in the later parts of my story (about three-thousand years later). One of those characters is a commander of an army and has learned that honesty is best in all things. As such, he never censors himself. He swears like a drunken sailor. A common expletive of his is "By Tolesh's brilliant fuck!" Tolesh is the goddess of life, from her the spark or 'soul' of all life in the world came forth.
I think those are the only ways I've used the f-bomb in my story. I usually try to make swears appropriate to the setting.
"Gehemol!" is similar to "Hell!/Damn!/Fuck!" It's the place below the world where they say the dead dwell. "Curse you to Gehemol!" "Ride Tralt's lhipossa to Gehemol!" and the like.
"Arse" usually replaces "Ass" or "Asshole." Such as, "Jykal, you're a clogged up herdsmoll's (lizard-cow-thing) arse."
"Jhulkos curse you!" is similar to "You're insane/batshit/crazy."
I even decided to make up an in-language term for penis/cock, amongst others. "Lhipossa." Given a scene where a rather immature main character is trying to get a rise out of a bard character standing in front of his tent in the nude and his bodyguard by saying "He has a fine lhipossa, what happened to yours? Fall off?" Also use it as a chapter title because I can get away with it. Frak/Frell and all that.
"Olthenna (death goddess) take you!" Is similar to, "Fuck off and die."
There's also a culture-specific expletive that really has no correspondence in modern language. "Toltenvaelk." It's an expletive when a commander's plans go to hell and they blurt it out as an order. Basically, everything is FUBAR so fight to the last person standing.
There's some examples. Pardon if they come across as a bit 'Whedon-esque.'