I run an online appointment scheduling software (http://www.appointmind.com/). The software stack is PHP and MySQL on Ubuntu. I do this alongside my my freelancer job.
I do Google Adwords (profitable, but no great volume), SEO, advertorials (profitable, too), guest blog posting (meh), Twitter, APIs (MailChimp, GetResponse), small business coaches, white label solution for partners, partners in general (web directories, homepage builders).
Getting sign ups is a very slow process. Can be frustrating from time to time.