I'd like to know if owners of sites like Onstartups.com and Startups.com have any revenue off of these websites?

and if not is there a possibility of having revenue from a site like this one?

I've been glancing at stackoverflow.com (listening to their podcast as well) and participating in both onstartups.com and startups.com - having worked with non-profits before, here are some potential revenue possibilities as a community of like minded folks get together.

1. Event / Conferences - It has already started with StackOverflow - where they have the DevDays - the market is currently recovering, but by the time everything is in full swing - both of these sites would have more content and experts than a First Tuesday event(VC/Startup matching). Sponsorships of a large community has it's season - and already at the DevDays San Francisco, Microsoft was offering free RAM upgrade to all the participants. When you have a community, it can be a very powerful movement.
2. Publishing - There are tonnes of startup books out there, but the pages here are curated, community maintained, edited. We are going to get to a point where there are some folks who just want to read the best of - perhaps on a Kindle or other devices or paper. There are already wiki-sites that collate the content to be printed on demand. Checklists, best practices, amazon linked affiliate accounts, case studies, et al.
3. Job Listing - StackOverflow have started monetizing this with their Careers section - hiring the right people for the startups - with a history of their response to company/business building and also their global network? Once you have the attention of startups and company builders - this becomes very compelling to business funders.
4. Partnership - YCombinator has their own community site, Hacker News for looking at responses of potential folks they would look to fund. I think both SO and HN are on the same path towards building up credible community with real discussions. HN's underlying software is open source (but it's written in Arc/Lisp) - and theirs surround themselves around news, tips, guides, and sometimes questions - having all these strong communities working together eventually would be very symbiotic in accelerating the training of entrepreneurs.
5. Training/Consulting - As the site's content grow, eventually its going to reach a point where someone new coming on board would have some difficulty finding its way around to absorb so much. Who knows, VCs may even start training some of their folks to have a reputation around here.
6. Market research - When you have a community sharing, you discover trends and patterns - one of the sister sites to stackoverflow has uncovered a market potential - scalable wifi services at conference events. Perhaps there would be an algorithm such as Flickr's interesting photos for this engine at a later time - where you get to see the attention of issues coming up. Once we have the critical participation point, we become representative of an emerging market and of course, we would have our needs. Being startup entrepreneurs ourselves, we are representative of the early adopters.
7. Advertising - Google adsense, relevant content services, CPA/Affiliate deals, PPC or CPM, as the site and content grows, the site can build up significant inventory and attention.
8. Exit Strategies - LinkedIn, WSJ, Thomson Financial, Reuters, The Conference Board - both Onstartups/Startups.com are democratizing the information of entrepreneurship and company building. The content of the site is user generated, the answers are user generated and the participants structure is meritocracy driven - Jeff and Joel are constantly tinkering with issues of some folks gaming the system (though the engine used here is currently forked from StackOverflow, and may re-align next year, according to their podcast) - once you have the momentum of providing value - I see it's something very familiar happening here as to what Clayton Christensen describes in disruptive technologies. We're going to move up the value chain very quickly.

While I probably wouldn't take medical advice from a site in this format yet, as we find members on these site that contribute their time and references, this is the best system I've yet seen to harness community knowledge.

Well I can tell you exactly what the deal is for THIS site!

1. You can only make ad revenue with tons of traffic. We hope to get that amount of traffic but it's not the goal. The goal is to create something of genuine value for as many people as possible, but the value is the goal, not the number of people.
2. (@fabien7474) By my calculations StackOverflow makes more money than that. They run ads based on # of impressions, not on # of uniques, so your math is way off. They have 1m impressions/day on weekdays; even averaging down considerably and multiplying by the CPM we paid at Smart Bear, you could be off by an order of magnitude!
3. We already know ads aren't the answer. The benefit is in developing the OnStartups.com platform in general as a destination for startups.
4. It's true that it's hard to cover even the $129/mo charge with ads until you have lots of traffic, so in general it's not a slam-dunk business model. However if you have a mechanism for getting traffic, it is cheaper than writing something yourself.

This is like blogging. It's almost impossible to make money off ads and blogging, even with thousands of readers (you need many 10s of thousands). But blogs can be a fantastic way to drive traffic to other things that do make money, like a startup or like consulting time.

Let's do some estimations:

For StackOverflow (a growing website) with more than 100K worldwide UV by day and 4M UV/month (see adPlanner stats ), one can pick a reasonable average CPM of 0.01$/UV/month then the monthly estimated revenue is equal to : 40 K$ /month. That is a pretty amount of money.

Thereafter, you can add every additional source of revenue given by Alex plus some other revenues specified in this thread.

And finally, you can conclude that StackOverflow is a running business (even if we don't know their operational costs...we can estimate them as well knowing that they are a company of 3 people).

Let's say that I totally disagree with Oleg's answer :-)

Concerning answers.onstartup.com, it is more difficult to guess since traffic is lower and monetization ways are not so manifold. But I don't think that it is a business losing money.

Answer to #1: I'm not an owner, but I don't think they do have any revenue yet.
Answer to #2: I think it's close to impossible. Your costs: monthly fee to Stack Overflow, hosting, some moderator(s) who keep the site clean and lean. Your revenue: ads? Forget about it. There won't be nearly as many pageviews required for ad revenue. Nothing stops everybody and his dog from opening an exact replica of your site, thus driving pageviews and potential revenue off your site.

I think that even Stack Overflow itself won't be a hot darling for a long time. It's relatively easy to develop. There will be competitors soon. Then there will be a sourceForge variant (i.e. free) too. End of opportunity.

Please take a look at Stack Overflow. Revenue is obtained through advertisements, sponsoring of badges and other means. For instance, a start up magazine might eventually be interested in sponsoring a badge here .. meaning the badge would become the "magazine name" badge instead of the "Taxonomist" badge.