NSFW is an abbreviation for the phrase Not Safe For Work. Please see this link for a detailed explanation.
Some of the comics linked from this site contain adult content. However, not all adult content is created equal. Rather than try to go into detail about each one in the tiny links frame I decided to link to this page instead, where I can go into detail. I'll list them by frame page and then by comic, though I do not guarantee that I'll always keep up when I move comics around. Click on the asterisk (*) next to the comic to go directly to the details for that comic. (If a comic has no asterisk it is probably PG or G.)
Rated PG, some violence and adult situations but you never, ever see naughty bits.
Occasional non-explicit nudity.
Nope.
Adult situations, very much so. We've seen the afterglow, we've seen the occasional sideboob or the curve of a buttock, but the comic has never shown naughty bits or the actual sexual act to date.
We have seen sex toys such as leather harnesses, gimp masks, dildoes and vibrators. We have not seen insertion.
There is homosexuality, one character is transsexual, and there are self-aware robots having consensual sex with humans. One character is a professional dominatrix, albeit only ever shown in street clothes.
The comic is not explicitly sexual, but YMMV.
Nope.
Some incidental partial nudity, obscured or lacking detail.
Nope. At most you'll see some nipple outlines through cloth. xkcd characters are drawn as stick figures so even if they're stark naked you won't see anything...
A bit of nipple on Evil Spinnerette in a recent story arc. I think. It's hard to be sure, but it's certainly strongly suggestive. You often see nipple outlines through shirts and the women tend to be large-breasted.
Trigger Warning
Collar 6 is suggestive, has strong S&M, bondage, and domination themes, and a fair amount of lesbianism. Some scenes are explicit, though we've not seen coitus in any form. There have been scenes of unwilling sexual participation -- I hesitate to call it "rape" without qualifiers, as the intent was not sexual in these scenes, but rather an attempt to alter mind states. It's difficult to explain without giving the story away. However, I'm also not willing not to call it rape, as there was definitely coercive sexual manipulation. For the record it was strictly female-to-female, and keep in mind that this is a fantasy and the action is in line with the story; and the setting is antagonistic -- bad guys doing bad things to good guys. It does not appear to promote rape or attempt to cast it in a positive light. That's all I'm going to say -- you'll have to judge for yourself. If it's likely to offend you, stay away.
I know some people are sensitive to such things (for good reasons) and I neither intend to judge them nor do I want to judge for them. I'm just giving fair warning.
Nope.
All the characters are anthropomorphics. That said, the artist doesn't really show naughty bits. If there have been any naked breasts they'll have been completely covered with fur. Definitely adult situations and some sex, but no genitals. Inter-species relationships, but of course they're all furries.
Moon Over June is unabashedly and blatantly X-rated including explicit lesbian sex and full nudity, bondage, genital piercings, and so on. You can't get more explicit unless you include goatse shots and gynecological exams. ...Erm, oh right. Oops. I guess you can't get more explicit.
Curvy shows full nudity and includes explicit heterosexual and homosexual sex acts, gender bending, some kink (e.g. bondage play). On the other hand, unlike (for example) Moon Over June, the sex is not the point of the comic. A major part of the comic, yes, but not the main point.
Nudity, explicit sex, masturbation, explicit language, adult situations, and so on.
Nope.
Occasional nudity.
Rare incidental nudity.
Nope.
Adult situations, suggestive dialog and mildly revealing clothing (no naughty bits) at most.
Very, very NSFW. Explicit sex including oral sex, penetration, body parts, body fluids, and orgasms. Leaves nothing to the imagination. That said, it's Elf Porn, but it's Sword and Sorcery Action Adventure Elf Porn. There's a story along with all the sex. There's just a lot of sex in the story.
"Mature readers." Some nudity.
Nope.
Partial nudity, adult themes. "Tits, but no bush" and you don't actually see nipples as such. May be NSFW depending on where you work.
Some female nudity. Enormous boobs, boob jokes, double-entendres and suggestive comments.
Marginal -- an unclothed magical clone of one female character shows for a time, made out of some jelly-like substance. Details (e.g. nipples) were omitted so it wasn't particularly racy. Some men in the story have been turned female (and some of them like it) and there's some nudity but the artist hides the naughty bits.
None that I'm aware of.
Nothing much. Occasional glimpses of partially covered female derrieres, scanty clothing, suggestive but not explicit.
Some nudity, some suggestions of rape, fetish, apparently homosexual characters. No explicit sex as yet.
Some partial nudity, breasts and buttocks. Adult themes. Trigger warning: rape on a psychic plane (mid-Nov 2013). That is, one character entered another character's head via a sort of astral projection and experienced rape in that medium.
So far mostly all we've seen is that the spandex-wearing main character's genitalia are rather prominent. The most we've seen is a roundish bulge in his crotch, and cleavage and (covered) erect nipples on some female characters. So far I'd rate it PG-13 at worst. "This comic is intended for mature audiences. It may contain content unsuitable for children."
Some partial nudity.
Scantily clad women, but that's about it.
Nope.
The title notwithstanding, no sex to speak of, lots of violence.
Nipple outlines and large breasts and bulging crotches. Secondary sexual characteristics are exaggerated to the point of caricature but it's all in fun. Adult situations, nothing explicit. However, there is lots of blood and gore. Harsh language is self-censored in a rather silly manner.
Nope.
Blatantly X-rated and deliberately offensive. Seriously.
Adult themes.
Occasional nudity, female breasts and buttocks.
Some nudity and some sex. The sex hasn't been explicit (in fact, we come in after the act) but there are some explicit indications. And you get to see naughty bits. Most of the comic isn't very explicit but there are occasional scenes. The sex is part of the story, this isn't a comic that's about sex for the sake of sex. However, you might want to steer clear at work.
Irreverent adult humor which may be offensive to some. Some nudity.
Nope.
Eve's Apple has some non-explicit sexual and adult situations and has strong homosexual and transvestite themes, in addition to the main character and at least one supporting character being transsexual. This comic is very much about non-mainstream gender identification. Some glimpses of nudity (female breasts and buttocks), adult situations.
There's a fair amount of partial and some full nudity, but the artist appears to take care that there is no full frontal nudity. You'll see tits and asses but no bush or wang (though you might see just a hint of pubic hair occasionally). Any full nudity takes care to hide the reproductive organs behind handy props or other characters. Most of it is in the third story.
Nudity, explicit sex, penetration, bondage, kink, suggestive dialog, gender bending, adult situations... don't look here for Ward and June.
Nudity and sex. Mostly female-to-female oral sex.
Nudity and sexual situations. Also very surrealistic.