I had an occasion recently where our website was compromised - a link farm was added to a couple of the pages on one occasion, and on another occasion, a large and nasty aspx file was put on the server. I won't mention the host's name (Hostway), but I was pretty annoyed that someone was able to do this. No, it wasn't a leaky password - around 10 sites hosted by HW with consecutive IP addresses got trashed.
Anyway. What I need is a utility or service (preferably free) that takes a snapshot of my websites contents, and then regularly monitors the files (size and datestamp) for unauthorized changes or additions, and alerts me. I've used web services that monitor one file for changes, but I'm looking for something a bit more aggressive.