It's surprised me that there's been very little discussion of The Long Now here on Less Wrong, as there are many similarities between the groups, although the approach and philosophy between them are quite different. At a minimum, I believe that a general awareness might be beneficial. I'll use the initials LW and LN below. My perspective on LN is simply that of someone who's kept an eye on their website from time to time and read a few of their articles, so I'd also like to admit that my knowledge is a bit shallow (a reason, in fact, I bring the topic up for discussion).
Most critically, long-term thinking appears as a cornerstone of both the LW and LN thought, explicitly as the goal for LN, and implicitly here on LW whenever we talk about existential risk or decades-away or longer technology. It's not clear if there's an overlap between the commenters at LW and the membership of LN or not, but there's definitely a large number of people "between" the two groups -- statements by Peter Thiel and Ray Kurzweil have been recent topics on the LN blog and Hillis, who founded LN, has been involved in AI and philosophy of mind. LN has Long Bets, which I would loosely describe as to PredictionBook as InTrade is to Foresight Exchange. LN apparently had a presence at some of the past SIAI's Singularity Summits.
Signaling: LN embraces signaling like there's no tomorrow (ha!) -- their flagship project, after all, is a monumental clock to last thousands of years, the goal of which is to "lend itself to good storytelling and myth" about long-term thought. Their membership cards are stainless steel. Some of the projects LN are pursuing seem to have been chosen mostly because they sound awesome, and even those that aren't are done with some flair, IMHO. In contrast, the view among LW posts seems to be that signaling is in many cases a necessary evil, in some cases just an evolutionary leftover, and reducing signaling a potential source for efficiency gains. There may be something to be learned here -- we already know FAI would be an easier sell if we described it as project to create robots that are Presidents of the United States by day, crime-fighters by night, and cat-people by late-night.
Structure: While LW is a project of SIAI, they're not the same, so by extension the comparison between LN and LW is just a bit apples-to-kumquats. It'd be a lot easier to compare LW to a LN discussion board, if it existed.
The Future: Here on LW, we want our nuclear-powered flying cars, dammit! Bad future scenarios that are discussed on LW tend to be irrevocably and undeniably bad -- the world is turned into tang or paperclips and no life exists anymore, for example. LN seems more concerned with recovery from, rather than prevention of, "collapse of civilization" scenarios. Many of the projects both undertaken and linked to by LN focus on preserving knowledge in a such a scenario. Between the overlap in the LW community and cryonics, SENS, etc, the mental relationship between the median LW poster and the future seems more personal and less abstract.
Politics: The predominant thinking on LW seems to be a (very slightly left-leaning) technolibertarianism, although since it's open to anyone who wanders in from the Internet, there's a lot of variation (if either SIAI or FHI have an especially strong political stance per se, I've not noticed it). There's also a general skepticism here regarding the soundness of most political thought and of many political processes. LN seems further left on average and more comfortable with politics in general (although calling it a political organization would be a bit of a stretch). Keeping with this, LW seems to have more emphasis on individual decision making and improvement than LN.