I use
www.outright.com and really like it. As mentioned previously, it is VERY simple to use. You enter your income, expenses and it will give you an idea as to how much you should send in for quarterly taxes. I really like it.
I started using this after getting frustrated with QuickBooks Pro. BUT I do use QB for invoicing and just keeping track of who owes me what. Whenever an invoice gets paid, I record it in QB as well as Outright.
Not the most efficient system, but it works for me at the moment!