Update 2010/04/25: A built-in feature like this seems to be around the corner. Note the "Linked" column on the right – this has been deployed exprimentally on Meta, but I suspect it won't be long before it's on all trilogy sites. So HREF Overflow is probably close to no-longer-relevant – thanks Jeff, I had just found the killer business model for making loads of money with this. No, really.
Update 2010/04/11: Now with data from the April data dump. This time, it also includes references made in comments (although you can hide these if you want to).
Update 2010/03/19: Since features like this have again been requested recently, I decided to finally update HREF Overflow. It's still based on the data dump, so no realtime data. But since I've simplified the process of updating the data, I'll be able to put the coming data dumps into it rather quickly. It now includes cross links between all four sites of the trilogy.
What is HREF Overflow?
When you have found an interesting question (say, "How do I do X?") on (e.g.) Stack Overflow, and you would like to read other similar question threads, one way to find such threads is to find out what other questions on the trilogy link back to "How do I do X". Which is what HREF Overflow does.
It shows you all* questions and answers on the trilogy that have links to the question you're interested in (or to one of its answers).
* All it knows about, that is. As mentioned, this isn't real-time data.
How can I use it?
There are two ways to query HREF Overflow. The first is to go to
http://hrefoverflow.appspot.com/SITE/QUESTION_ID
where QUESTION_ID is the numerical question id that appears in the URL of the question, and SITE is one of so, su, sf, and meta. For backwards compatibility, the SITE/ part is optional and defaults to so/.
The second way is to create a bookmark called "What links to this?" (or the like) that changes the location to http://hrefoverflow.appspot.com/referrer (there's a bookmarklet on the page that you can just drag to your bookmark bar).
When you request /referrer from HREF Overflow, it will pull out the information to the question you're coming from (unless, of course, your browser hides the referrer). You may also request /referer, so it's up to you whether orthographical or technical specs are more important to you.
