As designers, you are always looking for ways to be more productive and stay organized. A great free service that I use is Side Job Track. You can keep track of all your clients and the projects you are working on. You can also send estimates and invoices via email or print them out, and they can even be customized to look how you want. Unlike other services that have a free service but limit the number of clients and projects you can have, Side Job Track has unlimited number. If you use the service, you can even add your name and company to their home page and get a one-way link to your site. For anyone interested in seo, their PR is 4.
Go create a free account and check it out.