Greasemonkey is a brilliant plugin for Mozilla Firefox which allows you to run JavaScript snippets on loading certain webpages.
Using Greasemonkey, it is possible to embed OSM into other websites.

Several months ago, I originally started to embed Ordnance Survey maps into geocaching.com, since there was a reasonable demand for this functionality.
In the most recent version (after working on the OL codebase a little), I added OSM to the available layers for the map widget in an attempt to spread OSM's reputation among geocachers. The good news is that two people have already commented that they're surprised by OSM's coverage, and have started mapping their own data!

I gradually intend to push OSM further up the layer list and hopefully make it the default layer in future versions.