Inverting OpAmp: http://www.chem.uoa.gr/Applets/AppletOpAmps/Appl_OpAmps2.html
DAC (Digital-Analog Converter): http://educypedia.karadimov.info/library/26072009_303797.swf
Interesting java applets/animations:
http://educypedia.karadimov.info/electronics/javacollectors.htm
http://www.falstad.com/circuit/e-index.html#clockedsrff