I've been using a windows/console app, running it from the basic task scheduler. Basically, all it does is make an httprequest directed to the site/keepalive.aspx, and writes results to the application event log. It runs every 15 minutes. Doesn't do much, but it seems to keep load times to within better limits than not running it. Occassionally, it will log a timeout error, but so far, nothing that I can't tolerate.
Now, I don't know what all these 'pay-for' services do exactly, but I can't really see it worth $300 dollars a month, if that's basically what it's doing. I run my app from my workstation at work, during the office hours, and from home at night, so I've got coverage almost 24x7. I know if you've got commercial sites to support you may need a better server based solution, but PC's are pretty cheap, and if you've got a cable/DSL connection, it beats having to put out that extra money.
So, my question is this - Is that all these folks are doing, or is it more complicated than that? If not, I'd be willing to work with some folks to expand my little app to cover it. Payment would be up to those who find it useful (donation). Just let me know if you want to go that route and we can work it out together. Your thoughts?