We keep tab on our competitors, industry websites and other websites of interest using the following two tools. Both are free and both will email you alerts when things change.
Google Alerts
http://www.google.co.uk/alerts?hl=fa
Using this you can focus your query on a search phrase, result type (blog, video etc), how often to alert you, a delivery email address.
I find this tool great for tracking names of clients (e.g. Veale Wasbrough Vizards), prospects (e.g. Wedlake Bell) and phrases around the legal sector e.g. alternative business structures.
Follow That Page
http://www.followthatpage.com/
This tool allows you to track whether a page has changed or not. I use this for tracking changes on news based sites such as Legal Futures, Law Gazette so I know when they have updated their pages. I also use it for tracking when competitors have updated their sites.