Invoice2go - Full featured, invoice and estimate solution with full mobile support.
ChargeOver - ChargeOver automates recurring billing, subscription billing, & payment acceptance. A SaaS that integrates with Xero, Quickbooks, & has an API. Free trial.
Zoho Invoice - 100% free online invoicing software for small businesses.
Elorus - A super-easy online invoicing & time tracking software
BillQuick - BillQuick Time Billing Software & Project Management Software is an ideal solution for all Service Professionals. BillQuick will eliminate your billing pains.
MinuteDock - Web-based time tracking software designed to integrate with the Xero accounting package.