Most wikis maintain revision history for every web page in the system so you can easily visually compare changes between any two versions of the same page side-by-side. For instance, this colored edit log from Wikipedia shows how the page for Bush changed the day Obama was sworn in office.
Now if you like to have a similar change log for regular web pages as well so that you know exactly what text has been added, removed or where the edits happened since you last visited the site, check out Versionista.
Versionista is both a web page monitor and visual comparison tool. You supply a list of web addresses (or URLs) that you want to track with Versionista and it will send you an email alert as soon as the site owner edits something on the page.
What I like most about Versionista is the way it highlights changes in a web page (see example)- all the edits are color coded and both versions of the page are timestamped and placed side-by-side so you can quickly compare the old content with the new.
Other services that can help you monitor web pages include Google Docs, Change Detection, Watch That Page or Page2RSS - the latter helps you track changes on a page via feeds itself and is good for ego surfing through Google search.