I've been using Freshbooks for quite some time and have recommended it to many people now.  It is easy, simple to use, free for up to 3 clients.  They keep on making it better over time which is quite nice too.  I've made a lot more $ by having a system that tracks invoices, payments and all that (rather than the 'doing it by hand using Word' method I used to use).  It also supports recurring billing which I've found to be great for automatically generating ongoing web hosting invoices.  Creating a new invoices is very simple and intuitive.<p>Here's a link (referral) to check it out: <a href="http://www.freshbooks.com/?ref=e8ba201056138-1" rel="nofollow">http://www.freshbooks.com/?ref=e8ba201056138-1</a>