As of November 15, 2012, Google cut off support for Internet Explorer 8. After the "kill date," they expected some users in Internet Explorer 8 would have issues with features in various Google Apps (e.g., Calendar, Sites, Docs). Eventually, Google Apps will stop working entirely in that browser.
If this affects you, no worries! You can install Google Chrome or Firefox, and anyone using Windows 7 can install Internet Explorer 9 or 10, which are still supported by Google. (Thanks to Christa in ASR for this helpful info!)