Every one of the several hundred application notes from Microchip Technologies.
http://www.microchip.com/1010/suppdoc/appnote/alpha/index.htm
Application notes
http://www.microchip.com/1010/suppdoc/appnote/index.htm
A collection of application notes from various sources, mostly RF stuff.
http://rfengineer.cc/appnotes.htm
Analog, mixed signal and comparator application notes.
http://focus.ti.com/docs/military/catalog/resources/applicationnotes.jhtml?navSection=appNotes&templateId=5604&familyId=2§ion=products
Quite an extensive list, almost all of these are in Adobe Acrobat format.
http://www.mif.pg.gda.pl/homepages/frank/index.html
RS-485 communication application notes.
http://www.bb-elec.com/technical_library.asp
Application notes and other documents, complete list, discretes chip, digital mostly.
http://www.fairchildsemi.com/apnotes/
Chip Search source of IC information. You must register to use the resource.
http://www.icmaster.com/login.asp