Extracting frequencies from FFT I performed 512 point FFT on a signal. I got another set of  512 Numbers. I understand that those numbers represent amplitude of the various sine and cosine waves having different frequencies. 

If my understanding is correct can somebody tell me how to know the frequencies of those sine and cosine waves from the knowledge of those 512-numbers (i.e amplitudes) ?
Assuming your 512 samples of the signal are taken at a sampling freqeuncy $f_s$, then the resulting 512 FFT coefficients correspond to frequencies:

0, $f_s/512$, $2 f_s/512$, $\ldots$, $511 f_s/512$

Since you are dealing with discrete-time signals, Fourier transforms are periodic, and FFT is no exception.

Therefore you can think of your last coefficient as corresponding also to frequency $511 f_s/512 = (511-512) f_s/512 = -1 f_s/512$.

The same applies to the second to last coefficient, and so on. This is the mirroring commented by Daniel Hicks.

Also, if you are transforming a real signal, then all your information is contained in the first 256 FFT coefficients. The rest are simply complex conjugates of the first coefficients.