Absolutify is a WordPress plugin by Benedict Eastaugh which turns relative URIs in posts, pages and comments into absolute ones (see RFC 1630 for details of absolute and relative URIs).
To use Absolutify, simply install the plugin and instead of writing
http://example.org/my-link/ (where example.org is the address of your
WordPress site) in your posts, simply write /my-link/ (initial slash
required). The plugin will then resolve that relative URI to an absolute one.
If your content gets loaded in a context outside your site, such as a feed reader, then relative URIs won't work--only absolute ones will. Absolutify lets you use the convenient, shorter format of relative URIs, while ensuring that your links will still work in all contexts.
It can also be useful if you change the location of your site. For example, you might move from one domain to another, or move your blog from a subdirectory on your site to the domain root. Absolutify will ensure that all the links in your posts still work.