Hyperlinks in the form of web links are the lifeblood of today's internet and world wide web. Examination of your web server's log files, either directly via tools such as Webalizer or indirectly via analytics services such as Yahoo's or Google's can show you who's visiting your web site or blogs.
But who's visiting your site isn't the whole picture; following a hyperlink is an active process. To complete the picture you need to find out who's linking to your site, which is a passive process.
If you're running a blog you may be able to use trackbacks or pingbacks to find out when a site links to you, but only if the linking site support the trackback or pingback protocol and then only if this is enabled on both sides of the ping relationship.
So what about those sites which don't support trackbacks or pingbacks or who don't want to be discovered that they're linking to you?
That last use case may seem overly paranoid, but as Chris Heilmann recently discovered, knowing who's linking to you is less a luxury and more an essential piece of information that can reveal unwanted content placed on your server by a third party ... and third parties placing content on your server is never a good thing.
Working for Yahoo! and working with people such as Chris, who understands how the web and the net work, has taught me a massive amount over the last 4 or so years. So in this case the answer is glaringly obvious once you stop to think about it, the major source of who's linking to who is ... a search engine.
So now not only do I know who's visiting my sites but also who's linked to them, by using Google's Blog Search and synching that through Google Reader to NetNewsWire, my RSS reader on my laptop. The moment someone links to me and that gets picked up by Google's spider, I get an alert. Of course, this doesn't cover all eventualities but it's always a good thing to have more than one source of information, especially when that comes for free.
Written at home (51.427051, -0.333344) and posted from the Yahoo! London office (51.5141985, -0.1292006)