Web Design, Web Hosting, Graphics and everything creative.
Billing, billing cycles, add-hoc billing and invoicing and my own book keeping.
I have been using nBill for 4 years now and it has made invoicing and receiving payements very easy. It also looks professional and allows your clients to log in and see their past invoices.
Mainly because of its seamless integration with Joomla, but as time goes by the added functions have made it one of the most powerful and flexible invoicing/billing/bookkeeping systems around (I think it is not sold as a bookkeeping system but the way I use it has made my tax calculations at the end of the year a doddle).
The support has been great everytime I needed to use it, it has been stable and I use it on 4 websites, including the free version and some free versions on some of my client websites.