We had built a simple wordpress plugin to show contact us details on wordpress blogs. We made it as we wanted to show our AeroLeads details on this blog and decided to put it on wordpress.org so others can also use it.
We never realized that this plugin will get this popular with almost 2000 downloads in 2 months and will get so many queries regarding new features and customization. This also drives traffic to our main site AeroLeads too since the plugin name has AeroLeads in it along with we being Publisher url.
This plugin do drives good traffic to AeroLeads.com and I am sure has helped us to get some signups as well. I think for startups creating plugins and extensions can be excellent strategy to drive traffic (it is OK if it not relevant) as seen from other major sites. Moz, SEOBook, HubSpot etc has published many tools and plugins which result in bringing potential users to their main site.