Copernic
Customizable free metasearch tool which queries many engines, removes duplicates, and validates the results.
http://www.copernic.com/

Geowhere
[Win] Multifeatured freeware that queries multiple search engines at once.
http://www.geowhere.net/

FirstStop WebSearch
Customizable desktop software for Windows that searches multiple search engines and websites simultaneously. Freeware and trialware versions available.
http://www.firststopwebsearch.com/

JOC MP3 Finder
A free tool to search and download MP3 files from the Net.
http://jocsoft.com/mp3/

1-Click Answers
This free reference tool from Answers.com can look up quick answers in a range of authoritative sources, while you're working with any application.
http://www.answers.com/

Qube
Desktop client which offers browserless Internet meta-search. It searches any text already on screen (or manually entered) and retrieves collated results.
http://qube.qelix.com/v2/

PractiSearch
Free dockable web search toolbar that can launch searches in myriad search engines from many categories. Includes automatic translation and can be extended with more search engines.
http://www.practiline.com/practisearch.htm

Sekhol: Auto Web Browser
Gratis metasearch software that queries several web search engines, accesses found web pages, and crawls their links. Free download.
http://www.sekhol.com/

Telescope Search
Free metasearch tool, which queries several search engines in six categories, removes duplicates, and also supplies an option to check links.
http://www.alkalisoftware.ca/Telesc.html

MindRetrieve
Open-source, browser-based application that searches the user's history of viewed pages.
http://www.mindretrieve.net/

Foboz
A metasearch program with access to over 300 search engines. Selects specialized search engines based on each query.
http://www.marsjupiter.com/foboz