Semiconductor cross reference search. One of the most extensive cross reference guides on the net.
http://www.nteinc.com/cgi-bin/nte.pl
Alphabetical and search lookup for application notes.
http://www.rohde-schwarz.com/www/dev_center.nsf/dynframe?OpenAgent&navig=/www/dev_center.nsf/html/nav,10,11&content=/www/dev_center.nsf/appnotes?OpenFrameset
Application notes on transformers.
http://www.signaltransformer.com/signal/techlib/techlib.cfm
A great little resource for various application notes and data sheets, replacement guides are included.
http://www.st.com/stonline/books/
Technical papers, applications notes and product reports for various types of coax-cable.
http://bwcecom.belden.com/college/college.htm
Search for products, application notes, data sheets, tech briefs from Harris, Intersil and GE. Discretes and chips.
http://www.intersil.com/design/apps/index.asp
Application notes, product notes, papars, circuits, designs, and data sheets. an extensive reference for the RF design engineer or novice.
http://rf.rfglobalnet.com/library/home.htm
Datasheets for semiconductors, especially for obsolete parts.
http://www.chipdocs.com/