Yes, there's Yahoo and Dogpile, but I haven't used either of them in close to ten years. Google works for just about all of my general searches. But there can be more than meets the eye to using a search engine - for one thing, you have to check out the options and the "advanced" stuff.
Google has many options, as this page demonstrates:
http://www.google.com/advanced_search?hl=en
Also, the line across the top lets you search for, for example, the latest news on a topic or celebrity who's, for example, mentioned on the radio as being the latest jerk, or trending on Twitter and you want to know why. Under "more" there are other specialized searches. Groups, for what it's worth searches both "Google Groups" mailing lists/forums and Usenet newsgroups, conflating (?) them as if they were the same thing. If you don't know what Usenet or newsgroups are, that's okay...
IMHO, much of that "invisible web" isn't really hidden, it's invisible because most people only look at the first page of results or only look based on one or a very few keywords. Composing a good string for a search engine takes some finesse and thought. I often try several slightly different search strings if the earlier results don't show what I think is out there. Perhaps you've hear the term "google fu."
There ARE specialized search engines, sometimes called metasearch engines. One I've often used for buying books is:
http://bookfinder.com
It searches the most popular book selling sites and databases at once for a book title and/or author, including Amazon, Alibris, and Powell's, and returns the results sorted by price. I've located and bought many books by using it. Often Amazon's used sellers do NOT have the lowest price, and it finds books that aren't available on Amazon. There's a similar competing one at:
http://addall.com
There are other useful but very task-specific sites, such as - you're addressing a (snail-mail) letter or package to a US destination and you want to know the zip code. This webpage will tell you:
http://zip4.usps.com/zip4/welcome.jsp
For finding bottom-dollar prices on computer parts, this site is good:
http://www.pricewatch.com/ (buyer beware, the lowest price probably doesn't have the best service)
I think Google has some "shopping" option or whatot, but I first saw Pricewatch probably over ten years ago and I like the interface. In recent years it has gotten into general merchandise.
I'm starting to think I should write some article or blogpost on this topic ... it looks like I just did.