I have the google toolbar, which has a "search site" button. It looks like a magnifying glass with two eyeballs. If it doesn't show on your toolbar, you can add it on by going through Options. I can go to
http://blogs.savingadvice.com/ put my search in the search box, click the search site button and it will search within the blogs.
Alternatively, if you don't have or want the toolbar, you can also use google to search within a site from their main search page. Just use site:examplesite.com and then your search terms. No space between site: and the address. For example,
site:
http://blogs.savingadvice.com cheap
to find entries using the word cheap
I do this in Fatwallet also, because their own search engine is so terrible.