Harvest ( http://getharvest.com ) - Time tracking and invoicing for freelance work

SendGrid ( http://sendgrid.com ) - API for sending and tracking email

Lighthouse ( http://lighthouseapp.com ) - Issue tracking for teams

Trello ( http://trello.com ) - Task tracking, lists

Stripe ( http://stripe.com ) - Fast, easy payment processing

BundleScout ( http://bundlescout.com ) - Third-party library update tracking (shameless plug, but I use BundleScout at BundleScout)