Modern browsers like Firefox provides a searchbox (smart searchbox) beside addressbar. In this searchbox you can decide which search engine should search your query. The browser provides you a few search engines in the list and it sets one search engine as default, but you can set by choosing any search engine as default. And you can add many other search engines in smart search box of your browser. You can add or remove these search engines.

Which search engines to add and which not it depends upon your work or kind of business. I have added many search engines but only few search engines I use to search which best suit my needs

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Amazone.com:
If you are looking bye or sell something then Amazone is website is best for this all, you do not have to google for it because Google will refer you there. I never used it.

Answers.com:
Answers searches for questions or phrase and provides you references, where Google can not help you. Sometime I use it.

Creative Commons:
Creative Commons provides you filtered search results which you can use for commercial purpose or modify build and adapt.
Sometime I use it.

Cuil:
Cuil was developed to kill Google but it was not a solid product but it is also in my search engines list and sometime I use it too.

Delicious:
Search for something on Delicious is very good, surely you can find some good result or links.

Ebay:
eBay is also similar to Amazone, I never used it to search any query.

Flickr Tgas:
flickr is for image search, it searches your tag for related images.

Google:
I always keep open Google in a separate tab so most probably I don’t use Google here.

Gutenberg:
Gutenberg is a project to provide you free books, you can search Gutenberg Project to search free novels, stories ans many other books.

IsoHunt:
isoHunt is a Bit Torrent search engine, you can saerch torrents.

LinkedIn:
Linkedin is world largest professional network, you can search linkedin for professional peoples professional profiles and companies profiles.

Mozilla Add-ons:
Most frequently I search for Mozilla add-ons in smart search box.

One Riot:
One Riot is a real time search engine, it provides you real time search results.

SourceForge:
SourceForge is for open source programs which comes zero of cost yet very useful.

Twitter Search:
Twitter searches real times results from Twitter.

Wikipedia.org:
Wikipedia is worlds largest and free encyclopedia for references.

Yahoo:
Yahoo is another large and powerful search engine which recently acquired by Microsoft.

Yahoo Answers:
Yahoo Answers are asked and answered by people like you and me and very useful.

Youtube:
Youtube is world’s largest video library, you can search for videos here.

Bing:
Bing is another great search engine from Microsoft.

Here is the link you can add more search engine: http://bit.ly/1eIf7O