Ideally bootstrapped, and made by 1-3 people, but not necessarily.
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.
- Help Scout builds awesome helpdesk software
- Feedly a perfect RSS/feed reader
- New Relic the best APM (and more) platform
- Ghost Inspector is perfect for automated browser testing
- and Runscope for API monitoring and testing.