If you happen to wanna target your product at web developers (where your product happens to add value to their clients), how and where would you start your marketing effort?
I would start by hypothesizing who your ideal customer is. Eg. A contractor managing several rails projects at any given time. This doesn't have to be spot-on the first try.
Once you have an idea of who an ideal customer could be it'll be more clear how to reach out to them, and more importantly you can test your hypothesis -- does this type of person want to pay you money for your product?
For example, if your target is a rails contractor you can reach out to a job board, find a consulting shop, talk to companies who have paid a rails contractor, etc. Find a couple potential customers and ask where they would find your product. Maybe you want to advertise on the right websites, go to the right conferences, etc.
What you're probably going to find is that your product isn't quite right. Either it doesn't solve the real problem they have, or you aren't talking to the right people. At this point you try again -- come up with another ideal customer.
It is really hard to get your product-market fit on the first go. You should be thinking less on the mindset of "I have a product that adds value, who do I sell it to" and more on the "How do I find people with a problem I can solve". At least IMHO :)
To Greg's comment above, also look at agencies as prospective targets. They employ a lot of web developers who do a lot of work for a lot of companies. In that scenario you're really targeting management of agencies to make them aware of your product. If they support it then they can sell it to their clients. And in fact can expand their services to include your solution which could mean more sales, more revenue.
You'll still need to define your end target customer since agencies typically focus on specific industries / company sizes.