(Please help to improve giving suggestions)

- Mathematics -
http://mathworld.wolfram.com/
http://mathworld.wolfram.com/FourierTransform.html

- General mechanics technical library -
https://sdp-si.com/

- Free software -
http://www.cs.wright.edu/people/faculty/jslater/vtoolbox/vtoolbox.html
http://www.gnu.org/software/octave/
http://maxima.sourceforge.net/

- Vibration equipment and applied technical notes -
www.pcb.com
www.wilcoxon.com
www.kistler.com
www.bksv.com