I reprised the RSS tricks session at BarCamp Charlotte on Saturday, where a popular trick was generating RSS feeds for sites that don't provide them. I use this to keep up with companies who don't have feeds for their press releases; it's also good for finding out about the public launch of stealth-mode companies.
Here are the companies I've found who create RSS feeds from web pages.
Dapper can create feeds from sites without existing feeds, too, but it's more complicated than the change alerts that other services generate. Mozenda goes farther into data scraping with its commercial service, but it's overkill for basic site monitoring.
A site-provided feed is better than this scraping approach—especially when it comes to sites with rotating content (such as client quotes). But when there's no feed and you really want to know when new content is added, these tools are helpful for pulling data into an RSS-centric information environment.