When you make a link, there’s no guarantee that the link will work 5 years later. Sites change their URL structure. I’m finding that many of my blog entries from 2000 are invalid.
Sometimes you want to link to a concept rather than a site. In such cases, it’s better to link to a Google query.
For example, rather than link to a site that defines SVG, I could link to the Google search define:SVG.
Rather than link to a tutorial on Excel array formulas, I could link to the Google search excel array formulas. I could even link to the first hit on Google for excel array formulas, mimicking the “I’m feeling lucky” button. This may change over time, but 5 years from now, it’ll still point to the most relevant link.
To link to the Google query for “excel array formulas”, just link to the URL http://www.google.com/search?q=excel+array+formulas. To link directly to the first result, add &btnI=I'm+Feeling+Lucky to the URL. (Linking to A9 is simpler: http://a9.com/excel+array+formulas)
PS: An alternative is to link to a permanent copy of the page from the Wayback machine (it has copies of my page all the way from May 2001 to Mar 2005). (You can’t use Google’s cache. When the site changes, the cache will soon change. But it’s a good defence against site downtime. Manually doing this is a lot of effort. Ideally, future browsers will automatically take you to the Wayback machine or the Google cache. (The Firefox plugins ErrorZilla and CacheIt come close.)