Hi,
I use the Virtual Accountant at hookhead.com and I find it to be very good. It's designed for IT Contractors who have limited companies, but I am sure it can probably be used for any small business - Don't think it caters for sole traders though.
I am an IT Contractor and only have a small number of Sales Invoices to keep track of, typically one per month. The Virtual Accountant works well for my needs and since it's online, I can run my business from anywhere. It handles Sales Invoices, Purchases Invoices, Expenses, PAYE/PRSI and VAT by the way.
I believe they will also give you a free trial if you ask for it. Don't say that I told you though ;-)
Bill