Billing clients is an important part of many online businesses. A respectable and reliable online invoicing solution is required to keep your payments coming in. Here are some of the invoicing programs available.
ModernBill's response to bugs can be rather slow due to the small development team (1 person). I have read where customer's were not happy with their slow response to bugs.
FreshBooks (formerly 2ndSite) turned a bad situation into a better one when they had a days downtime due to their RAID having problems. Customers appreciated the way they handled
Other online invoicing solutions are available from BillMyClients, BlinkSale and WHMCS.