[adventured]

I use one of these for various side tasks, because I got sick of having to run cron jobs for servers that I might switch between or kill off, and I run a lot of cron jobs. I also like the idea of offloading the cron hits to an independent service, so my servers focus strictly on more important tasks.

http://cronless.com

It's the first one I found that I liked and was reliable. It's very modestly priced as well.

I wouldn't suggest this for passive income however, as cron is a very low cost, value added type product. It's difficult to charge much for it stand-alone. You'd want to bundle a suite of services, like ping / uptime, cron, etc etc. into one.