Time. I saw something recently that made the point that "founder bandwidth is the most precious resource at startups," and I believe it. There's a nearly infinite amount of work to be done, and a fairly limited amount of bandwidth to go around between the founders, as we're doing the self-funded / bootstrap / still-working-day-jobs route right now.

That said, beyond time, then feedback and introductions are definitely huge. Of course that all ties together, as the time I spend networking trying to meet people I need to build relationships with, is time I'm not writing code, and vice-versa. It's a non-stop battle of priorities and a never-ending stream of trade-offs and judgment calls.

I know there are services that specifically target these needs, but I wonder if there are more opportunities out there.

Not sure... I'd guess there are quite a few free/low-cost SaaS apps that many (most?) startups could gain some value from using. Steve Blank keeps a list like that on his blog somewhere, FWIW.

Personally, something I took a flyer on recently was building a simple web-based tool for managing competitive intelligence. We'll be entering an established market when we ship, and there are plenty of competitors out there. Of course we think we have a unique spin that will give us an advantage, but I've just recently decide to sit down and do some serious competitive analysis and start paying a little bit of attention to "the other guys." I needed a way to keep all that data organized, so I built a one-off tool for it. I'm actually toying with making it open-source and/or launching it as it's own SaaS offering, but I want to play with it a little more first, and maybe do a "Ask HN" to see if anybody is interested in that sort of thing.