sure, i do it. Develop a custom system for a company and then charge a monthly use and/or maint. fee. If the original development was tested well you are essentially getting free money every month.

I have a full time job but one client on the side. I built a system for them that I net $1,250 per month on and do 2 hours of work a month at most. Rackspace takes care of almost all host related items for me.

I have a partner who also makes $1,250 from the same contract. In hind sight I could have easily done the whole project without him and be making $2,500 per month.

Been going for almost 3 years now, probably has a shelf life of another 7 years.

If you can get a few customers like this, you are set. The challenge is of course to get more customers, let me know when you solve that issue, I'm still working on it :)

edit: changed gross to net