After a long time, I found something really great and free of cost and more than that it's one of the most demanded service/hack/whatever by the community. Mandeep's http://www.dnnmonitor.com now offers free ping service to keep your DNN website alive and in memory. It requires you to register only a free account and then let you add your websites and does the rest of good work itself. The website allows only the DNN websites currently. The best thing I guess is to give the url of your keepalive.aspx file. I've not yet seen the reports/graphs that they provide based on the hits to your website.
I remember the times when we used to use the desktop utilities like Renegade Mind's keep alive utility for DNN but the major drawback of it was, if the computer where its installed shuts down or goes out of internet connectivity then your site is gone again :S
I hope the service will remain free forever :p
