Time spent with fiction when it's about some coherent body, be it a video game, book, tv series etc. Usually, the more time you spend with that coherent fictional body, the more immersed you become which means you can enjoy it more.

Railroads in Monopoly.

Railroads in an actual railroad monopoly.

Time spent with individuals - I'd rather spend time with friends than strangers.

Lots of things have increasing marginal utility at some hypothetical margin. But very few things have increasing marginal utility at the margin on which they are utilised, precisely because if people notice that increasing marginal utility, they will increase their consumption, until they hit a new point on the utility curve where the marginal utility is no longer increasing.

For example, shminux, above, talks about education. We can well imagine that education has steeply increasing marginal utility at some levels; once you have made the investment in learning to read, using that knowledge to learn some more things is very cheap compared to the benefits. But people are already aware of this, and so have already acted to do far more than just learn some basics, to the extent that, at the margin, educational consumption appears to be a costly signaling race.

I am not sure about the attention example, there looks to be an issue with units. For example, if we think in terms of percentages, going from juggling 10 things to 9 gives ~11% more attention to the nine remaining things. Going from 2 things to 1 gives 100% more attention to the remaining single. And that's just math, not increasing marginal utility.

And if we're talking about resources to be amassed by societies, pretty much anything with a network effect qualifies.

Multipurpose components, be they Lego, 80-20 pieces (the industrial version of Lego), electronics components, or disk space for a computer program - the number of things you can build from them grows rapidly as the number of them you have available increases, until you literally have more than you know what to do with.

Intelligence, on both an individual and societal level. Fooming AI is based on that idea. However, increasing the amount of this resource is a hard problem.

Perhaps rationality?

The early stages of any new thing with a lot of potential will behave that way, not only by networking effects, but people figuring out better ways of doing whatever it is, until both aspects reach saturation.

For every thing with increasing marginal returns, is there a saturation point, and what does it look like?

Both your examples are actually just about diminishing marginal penalties as you add more attention demands, moving away from 1, or as you add more defections, moving away from 0. The real question is whether there's a resource with no natural maximum that increases in marginal utility; and this shall perhaps be difficult to find.

I am not sure that trustworthiness has increased marginal utility. Think about ebay or Amazon, what is the difference between 99% positive and 100% positive. Or 97% positive or 100% positive. It would seem to me that with trustworthiness there is a tipping point, at which there is a huge spike in marginal utility, and all other increases don't really add much utility.

A related concept is that of the threshold good. (Perhaps someone with more economics schooling can help out with the formally correct term.) It's something that is useless until a certain threshold amount is obtained.

An example is the length of a bridge. A bridge that goes 90% of the way across a ravine is not twice as good as one that goes 45% across. Both are equally useless (for most purposes). Another example would be the stones in an arch--the final stone, or capstone, is a sine qua non.

The existence of threshold goods is what motivates the concept of assurance contracts, according to which people pledge money iff enough other people pledge enough money to get a project done.

But cheating on spouses in general undermines the trust that spouses should have in each other, and the cumulative impact of even 1% of spouses cheating on the institution of marriage as a whole could be quite negative.

In the comments on Scott's blog, I've recently seen the claim that this is the opposite of how traditional marriage actually worked; there used to be a lot more adultery in old times, and it acted as a pressure valve for people who would've divorced nowdays, but naturally it was all swept under the rug.

Time spent doing any kind of work with a high skill cap.

Edit: Well, okay not any kind of work meeting that criterion, to preempt the obvious LessWrongian response. Any kind you can get paid for is closer to true.

Knowledge, esp. math knowledge. It is difficult to measure the amount as well as the benefit, but it feels like one additional year of math education (which builds upon previous math knowledge) allows to modell (and thus understand in depth) significally more phenomena and structures than the previous years.

The question may be how valuable this ability is. I get the impression that it significantly simplifies understanding conrete practival domains (which can be modelled by the math in question).

This is related to the more general education comment.

A society where people abide by contracts 80% of the time is not 80% as good as a society where people abide by contracts 100% of the time; most of the societal value of trust (e.g. decreasing transaction costs) doesn't seem to manifest until people are pretty close to 100% trustworthy.

I don't agree since a society without contracts would be very, very bad. Still you ask an overall excellent question.

uranium

What other resources have increasing marginal utility?

Matter, if negentropy translates to utility by more than its square root, for example if negentropy translates linearly to increased lifespan and/or population, and we value lifespan/population linearly as well.

How undervalued are they?

I'm guessing that most people do not realize the above, and therefore underestimate just how high the maximum utility of the universe can be.