Hi All,
We have released a social media management and online brand reputation monitoring script in php. Our target audience is certainly - seo/ sem/ internet marketing agencies.
What will you recommend - how to reach those potential customers efficiently?
You can try local listing sites to reach SEO/ internet marketing companies. All most every company register their name with them to get business and for foung on internet easily so this is the easiest way to reach them.
The best way to get targeted audience is to provide reliable services to the customers who are already availing your services so that they can recommend you to others and you will get definitely get more traffic.
As you are looking for SEO companies, you should be easily get a list of companies in good directories like Yahoo or Dmoz or quality local Yellowpages sites. The companies which you are looking for, definitely might have worked for their popularity, so you will get them easily by little searching. 
It highly depends on your budget. If you have enough budget, you can find some good companies even by searching in Google. I suggest you to search in Google for “search engine optimization” or seo and then its clear that the companies which can rank their own website in top results of Google for such a competitive keyword “search engine optimization”, can rank your website too for your desired keywords. But these companies usually ask for a high budget i.e more than $1,000 per month. After choosing your company, search to read reviews about that special company and then you can decide. But be careful because there are many websites and persons do this but all of them are not good. As i said, choose company according to their own ranking in Google results.
The easiest way to market your product is on forums dedicated to seo, even if you cover BHW, DP, WF and sitepoint forum (how can you forget this ) you can get good no. of potential customers. And its very simple. Post a thread, give few (3-4) review copies and your product will get into rolling. Also there are many FB groups on seo. Post some article there and see whats the result.
You can search local listing sites or online directories.