I mentioned this custom search engine in my blog post yesterday. While it still has a long way to go, it already produces useful results, especially for OD&D, CD&D, and AD&D. This search engine only looks through a small set of carefully selected sites that have mainly older RPG content. You can search for "D&D" and not get mainly D20 edition results, for example.
When it is a little further along, I will integrate it into this site. For now, however, you can try it here:
RetroRoleplaying Search Engine