A couple of years ago, there were only a few players in the online invoicing world (freshbooks and zoho invoices IIRC).
But now I’m looking for an easy and well integrated solution, there are (too) many SaaS offers from expense management to invoice generation or accounting solutions. And they even integrate online payment (like paypal), customer access, multiple currencies, …

Here is a first selection of online invoicing systems:

• https://www.zoho.com/invoice/
• https://www.zoho.com/books/
• http://www.freshbooks.com/
• http://curdbee.com/
• http://www.getharvest.com/
• http://lessaccounting.com/
• http://www.roninapp.com/
• http://www.invoicera.com/
• http://www.getballpark.com/
• http://www.blinksale.com/
• http://www.clearbooks.co.uk/
• http://www.simplyinvoices.com/
• http://outright.com/
• http://waveaccounting.com/
• https://indinero.com/

As I said zoho and freshbooks are the most know Invoicing as a Service – IaaS providers. And the new zoho books provides unlimited invoice with a full accounting solution.
Curdbee is cost effective solution that offers the right essential features.
GetHarvest and Ronin provides project, time tracking, team management related features, which can be an important factor for you.

Last but not least, you should always look for a way to export (and import) your data in order to avoid customer lock-in. Off course the price can maybe help you, but most offers are below 25$ /month…

I must confess I’m a bit lost with all these solutions, so I’m open to any feedback.