I gave CurdBee a quick run through and found that it covers the basics of invoicing and payment reasonably well. I was able to get up and running quickly and found it easy to get test clients and service items entered for my first invoices. It hooks into Paypal Standard and Google Checkout for payment processing and also supports multiple currencies.
Be aware that there are no time tracking features and the overall functionality is sparse compared to more fully featured services like FreshBooks or Cashboard. It is also missing a lot of the nice “usability touches” that a more established product will typically have.
But if your needs are simple and you just need to send out quick invoices to your clients, CurdBee might be a good option for you. Accounts are currently free with no apparent limitations.
Are you using an online service for your billing? Could CurdBee work for you?