You don't strictly need a tool, especially if you work alone. However, I'd suggest you to use a CRM software to keep track of all the interactions between you (and your co-founder/employees) and your prospective customers.
As XS Direct pointed out, Salesforce.com is the usual suspect. But it is also expensive.
A good free alternative is SugarCRM. It is sold as a service, but the software is open source so you can actually download it and install in your server. The downside is that it is a complete CRM solution, and it can be difficult for a non-marketing expert to understand the terminology used and its functionalities.
The good thing of SugarCRM is that it allows you to do many things, like (of course) storing customer data, interactions with them and creating e-mail campaigns.
ZohoCRM is an alternative, as it offers a free version, but I personally haven't tried it.
Other tools you might find useful:
- Mailchimp to create newsletter and e-mail marketing campaigns. It isn't a complete CRM, but it's a pleasure to use, and it's free up to (if I remember correctly) 1000 email addresses
- Linkedin to scout for potential customers, even if it isn't exactly designed for that; Hoovers is a very rich (but quite expensive) directory of companies you can search by area, number of employees, market