I understand that when you have an app which you launched an year back or so, its pretty easy to get testimonials from your users which then works as 'credibility indicators' when published on your website. They work in your favor and convert more visitors into your customers. But I want to know if there are any credibility indicators which you can use with a new web app or website?
Supposedly trustmarks actually do have some influence on consumer confidence. Though all the whitepapers I've seen that claim this are also published by VeriSign and similar trustmark vendors. However, if you trust these claims, then a VeriSign, BBB, as well as a trustmark from your SSL vendor (perhaps an EV SSL cert) might help establish confidence.
Though to be honest, I've seen a lot of really sketchy sites that bear a ton of trustmarks, so I don't put much stock in them, especially as many dedicated trustmark vendors don't actually vet their trustmark recipients, and there's no way for consumers to report trustmark wearers for bad business practices. However, with BBB and VeriSign, there is at least the brand recognition factor.
Which brings me to the next method: brand recognition. If you can get some major clients early on, then you can display them prominently on your front page. This was done by Haulix when digital promos was still a very new thing. They had the major labels signed onto the service from the get go, which gave indie labels the confidence that reviewers and other press contacts would actually use the service if they signed up for it.
Likewise, if the app is developed by a well-known software publisher, or made by the same people as an established app, then that would be another way to bank on brand recognition.
Alternatively, you could send invites to notable internet personas and web press so that you can get their endorsements to put on the homepage. There are many news sites out there that cover web apps and new web startups, and an endorsement from them can go a long way for many different reasons.
Lastly, your app should be well designed. There are a lot of apps out there that might be quite useful and a great value, but they don't get much use simply because the app looks crappy and amateurish next to professionally designed sites like FourSquare, Twitter, Amazon, etc. And a poor design isn't just a lack of marketing sense, it can be indicative of a true lack of professionalism and competence. Many such sites have poor UX design and potentially even more serious problems. I mean, it's a bit like seeing a restaurant or store that has a sign made from spray-painted cardboard. If they can't afford professional signage, then who knows what other corners they're also cutting.
Even though many sites like Google, Amazon, Craigslist, etc. have succeeded despite their poor initial designs, those days are long gone. There are plenty of professional web app developers out there today like 37signals who know how to deliver, not just useful features, but a polished, well-designed user experience. So if your web app is poorly designed, users will just move on to a product from a more professional web studio.