iStat menus
It is a freeware, so download it at http://islayer.com/apps/istatmenus/. Do remember to donate if you have a spare change!
MenuMeters
MenuMeters is released under GNU GPL which means it is a completely free software. Unfortunately, the last update for this software was in January 25, 2006, a little over 3 years as of today's writing (2009).
However, if you are a proponent of open source and free software, you can download it at
http://www.ragingmenace.com/software/menumeters/index.html
Again, if you have a spare change, consider donating to the author of this software.
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On to the next topic, the maker of iStat menus also has 2 different widgets for your Mac OS X dashboard.
i) iStat pro
Download it here at http://www.islayer.com/apps/istatpro/
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ii) iStat nano
Download it here at http://www.islayer.com/apps/istatnano/
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Happy tweaking your Mac OS X!!
