Free Alternative for Visio

OpenOffice.org Draw
http://www.openoffice.org/product/draw.html

Inkscape
http://inkscape.org/

Dia
http://dia-installer.de/index.html.en

Network Notepad
http://www.networknotepad.com/

Freemind 
http://freemind.sourceforge.net/wiki/index.php/Screenshots

Poseidon for UML
http://www.gentleware.com

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Reference
http://forums.techsoup.org/cs/community/f/27/p/10800/10800.aspx

http://en.kioskea.net/faq/1400-free-alternative-to-microsoft-visio