Arbitrage is done my big financial institutions. It is simultaneous selling and buying of two similar financial products to take benefit of difference in prices.
Though arbitrage is no more popular now, it was very popular a few years ago. Trading volumes were less so arbitrageurs took a lot of benefit. However now trading volumes have increased and there are a lot of systems that make sure there is not too much volatility in markets.
But still arbitrageurs exist.
Some traders use automated trading systems to arbitrage.