WebMon is a tiny free tool which can monitor specified web pages and let you know when they're updated. Which means you'll be alerted just as soon as your favourite sites have something new to offer, without the hassle of having to keep revisiting them manually.
This all starts with a right-click on the program's system tray icon. Click "Add Page", enter the URL of the page you'd like to monitor, and how often it should be checked (you're able to specify anything from minutes to days).
Click OK and the program will then run in the background (it only consumes around 6.5MB RAM, so shouldn't be too much of an issue), accessing the specified page at your chosen frequency and raising a pop-up alert if it sees the content has changed.
Need to monitor more pages? You can manually add as many as you like (although, of course, if you're checking 500 every minute then that's going to generate a lot of network activity). And the program can also directly import your Internet Explorer Favorites, or URLs from a Bookmarks.html file.
Some pages will raise alerts all the time, perhaps because they contain dynamic content (current headlines, say). WebMon does a reasonable job of catering for this by allowing you to specify an area of the page to check, though. It's not easy to set up - you must wade through page source code - but still a reasonable solution for a free tool of this size.
And if you don't like some other aspect of WebMon's operation, the good news is that the program is highly configurable. Not happy with the default pop-up alert, for instance? Then you can change its position, have it close more quickly, replace the pop-up with a sound, even just run another program of your choice whenever the content of a web page changes (which presents all kinds of automation possibilities).