I really like Stripe (http://stripe.com) and have always been a fan of Campaign ...

I really like Stripe (http://stripe.com) and have always been a fan of Campaign Monitor (http://campaignmonitor.com). We've been making lots of subtle design iterations to Mailgun (http://mailgun.com) recently too.

Also I'd recommend checking out LeanStack.io (http://leanstack.io) which has a great list of SaaS companies. You might find some inspiration there.