Drip (getdrip.com) is really phenomenal in terms of amount of functionality and polish for a small bootstrapped team. Ditto Postmark (an entirely different par

Drip (getdrip.com) is really phenomenal in terms of amount of functionality and polish for a small bootstrapped team. Ditto Postmark (an entirely different part of the email problem space, incidentally!) ChurnBuster and Stunning have a stupidly high amount of business leverage to lines of code required to operate the business, and are among the best SaaS-for-SaaS you'll find out there. Visual Website Optimizer (VWO) might be the most impressive SaaS shipped by a bootstrapped solo (originally) entrepreneur that I'm immediately aware of. The current version is also pretty well done, especially compared to the general level that marketing-oriented software operates at. Products which are just great products: MailChimp, TaxAct, Honeybadger, Dead Man's Snitch (does one thing well), etc. I collect SaaS like some kids collect baseball cards so I could go on for a while, and don't mean to slight the two dozen or so other ones I'm passing up here. Tried to skew towards more niche things that many HNers wouldn't have used before; I mean, you already know Gmail and Basecamp are great.