The great thing about WinPYME is that it covers every management need of any company: invoicing, storage, income, expenses, projects, orders, profits, reports, and so on. Plus, it'll help you keep track of the costs of labor and products used on a job.
Another aspect that stands out is that it's incredibly easy to use, offering immediate access to the different company organization areas in its various tabs. In turn, this makes more complicated calculations much easier when figuring out invoices, expenses, etc.
In short, WinPYME is probably the simplest, most intuitive, and most powerful program of its kind on the market.