Creating your own search engine is all the rage right now - especially since it has become pretty easy to accomplish. It's also a great addition to your subject-specific blog or webpage; offering an "outdoors" search engine on your hiking blog would definitely make me think about visiting again.
Google, naturally, went full-scale customizable with Google Co-op. Co-op requires several easy steps, and offers a variety of options for optimal customization. It's very flexible, and very "Google-esque." Ninja, the search engine Josh wrote about a few days ago, is a Co-op engine. You can work the search into your page seamlessly, and if you are web-app savvy, you can do a lot with this tool.
If you are looking for something a little faster to throw together, a little easier in general, try out Rollyo. Rollyo does the same basic thing as Google Co-op - assembles a search engine based on chosen sites - but it does so with less fuss. And, of course, fewer options. It isn't (yet) customizable in terms of visual presentation, and it lacks some of the fancier options of Co-op, but I like it just the same. It also offers a "dashboard" which allows quick acces to all of your search engines, and a Firefox browser option. Has a bit more of a social-web feel, and is definitely an on-the-fly app.
Check 'em out - all the cool kids are doing it.
Oh, yeah, this is the first libraryland post since the merge. Links to Google Co-op and Rollyo will live on the Search Engines page for easy access and browsing.