Wolfram Alpha is great with electrical calculations as well

http://www.wolframalpha.com/input/?i=300+ohms

it'll show you the colours

http://www.wolframalpha.com/input/?i=5+volts+300+ohms

automatic calculations

http://www.wolframalpha.com/input/?i=+30+amps+300+ohms

another calculation

even filter time constants

http://www.wolframalpha.com/input/?i=1+ohm+1+farad

and look

http://www.wolframalpha.com/input/?i=1+ohm+2+farad+3+henry