Over the years I've done plenty of consulting, and I've always loathed every billing solution I've ever used. Either they locked me down to one machine (usually a Windows one) to do my billing and invoicing chores, or were labor intensive to a degree that was totally unnecessary.

Freshbooks is a breath of fresh air for the invoice weary consultants of the world. A simple interface allows you to enter invoice data, track payments and late paying clients, and even resolve disputes. Its an invoicing service that makes me want to generate a billable event, just to try it out!

A free trial allows you to manage up to three clients, and send unlimited invoices. The basic plan allows 25 clients for $14/month, which may be more expensive than some other solutions but, offers a great deal of flexibility that other options can't. Freshbooks even has plans to allow consultants to send paper invoices via snail mail, for your fussy paper-obsessed clientele.