Relative Strength Index (RSI) is most popular indicator. RSI is an oscillator which is oscillating between 0 and 100. The RSI is indicates the strength of trend:
values above 50 indicate a bullish trend
values below 50 indicate bearish Forex trend.
The RSI measures momentum of currency.
The centerline for the RSI is 50, crossover of the centerline indicate shifts from bullish to bearish and vice versa.