I have professed my love for Simple Invoices previously on this blog. Simple Invoices is a free, open-source, web-based invoicing system that can be installed on your desktop or web server. In fact, I use Simple Invoices on one of my businesses.
Today, I started using the Quotation feature. Quotations are not much different than Invoices. The only difference that you save the form as a Quote instead of Invoice. The default selection is Invoice.
A quote can be easily converted later to an invoice later by simply changing the category. Simple enough. If you need an web-based invoicing system, be sure to check Simple Invoices.