Basically, the idea behind the foundation is that we need to counteract the foreshortening of the future; in the past few decades, people's attention has gone from anticipating several decades into the future (eg, Orwell's 1984 or 1950's depictions of "the year 2000") to the next few years, to, essentially, only the here-and-now. Danny Hillis, the foundation's founder, says something I found rather profound and interesting:
I think of the oak beams in the ceiling of College Hall at New College, Oxford. Last century, when the beams needed replacing, carpenters used oak trees that had been planted in 1386 when the dining hall was first built. The 14th-century builder had planted the trees in anticipation of the time, hundreds of years in the future, when the beams would need replacing. Did the carpenters plant new trees to replace the beams again a few hundred years from now?
Plus a neat diagram illustrating the difference between "now", "nowadays" and "the long now", as seen by the foundation.
Now, of course, whether or not you agree with the foundation's purposes or you think they're a bunch of crackpots (or conmen), there are some really neat ideas: a clock built to last 10,000 years, chiming once a century. The Rosetta Project, an effort to catalog, record and permanently store, on optical disks that are human-eye visible (with a microscope... the point being not based on any computer language or operating system, etc.), all the known languages on Earth; (This isn't likely to happen, of course, since many languages are quite near extinction and documentation takes a long time... nevertheless, a good effort could be made); a permanent of human knowledge; and possibly more. There are even some actual benefits right now: the Rosetta Project is a live database, and though I haven't explored it thoroughly, it looks somewhat promising; there is also the Longviewer (aka the Timeline Tool), an Open Source timeline software tool. Looks interesting, and is supposedly running on the Biotechnology milestones page. There's also supposedly a permanent email server of some type.
Besides... I really really want one of the Rosetta Disks... it looks cool and I could be a keeper of a really neat (and important!) piece of history.