http://www.freshbooks.com/
FreshBooks is an online time tracking / invoicing system. It will generate invoices, accept payments via PayPal / Authorize.net / etc. It sends reminder emails and so on. Packages from $14 to $150 per month.
http://www.mint.com/
Mint is an auto-book keeping service. Give it your (US only) account nos, etc and it will track all your income and expenditure. Nice.
http://www.thisispearl.com/
Pearl's pitch is appealing. Integrate all your web services into one clean package. Let pearl handle CRM, accounting, billing, etc, etc. Feels a bit out of date, trying to be all things to all people. Interesting nonetheless.