We've had some success with this type of thing, especially in more niche business markets. Business software usually has a much greater lock-in factor than consumer sofware, so you often have to worry less about retention than you do in most consumer viral apps, which makes the effect even more powerful. If you augment the basic principle with other things (video, etc.), it can really go crazy.

An example: We have a product that performs a type of very specialized financial analysis in an automated fashion, for free. The analysis was normally done only by real estate and tax attorneys during specialized commercial property transactions, at steep hourly rates. [The analysis itself is a very rarely needed thing for very obscure clients...so we are not operating on a "try one, buy more" basis. We just use it as a loss-leader intro for some of our other services. It was just supposed to be a 'show off' piece.] Our initial marketing campaign for the service fell flat, because people simply didn't believe we could do what we said we could do. Both the automation and the free aspect were so unusual that it sounded "too good to be true," so people thought it was a scam of some kind. We're also not attorneys or tax professionals (although we have some on staff), so we lacked credibility. And why would we give away a multi-thousand dollar service - that the rare client may only need once - for free?

We started adding an "Analysis completed by EXXXampleService.com" statement to the bottom of the tax/legal documents in this market that we handled in our own business, and to those that we provided for law firm clients on an outsourced basis. We also added the same statement to the bottom of outgoing emails that dealt with this part of our business. Importantly, the statement in the email was a hotlink to a <2min video that explained what our service did in the most general terms and why we did it. Below this, there was another link to a longer-form video that explained the analytic process in more detail and showed the before and after result.

The uptake on our service surged after we implemented the document statements and email links. A large number of potential clients ended up viewing the longer-form video. We now have about 22% of all clients in the US that need this service performed. Which is still a very small market. And we don't make any money on this initial analysis. But - and this is the viral loop aspect - now that we have these clients, all of their legal and tax documents for the transaction are already in our document management system. And these documents have to be used by both parties in the transaction. And we have some enticing features built into our system that can be used for most types of these transactions, including 'normal' transactions. So everyone involved in the transaction ends up trying our software for "just this one specialized deal" because the docs being there will save them a tiny amount of time...but once 'inside' our collaborative dealroom software they realize that they simply can't live without it for all of their everyday deals because it saves them so much time and money. The end result is that we end up getting both parties to these commercial transactions as long-term clients. And we often sign up other third-party professionals that were also involved in the transaction (legal, accounting, environment consultants, etc.) for other related services, as well.