Why have Central Banks introduced Quantitative Easing? What other policies have been introduced in the 21st Century?

Central Banks have introduced Quantitative Easing (QE) as an additional mechanism to stimulate inflation in the economy.The two mechanisms that drive inflation (theoretically as this has not been proven as of yet) are:
Central Banks (CB) purchasing of government bonds from corporations helps drive down yields (coupon/price), thereby decreasing the cost of borrowing for corporations. This stimulates spending as they can borrow and service debt more cheaply. They are more likely to spend when they have more disposable income and therefore this drives inflation up.It also increases money supply in the economy when the CB buys government bonds with newly created money. This drives down the value of the domestic currency against foreign currency, increasing demand pull and cost push inflation.Finally QE discourages as savings rates for banks are driven down and costs of borrowing are passed onwards. Savers' savings rates fall, which discourages saving and incentivises spending as they receive less interest on their savings.<br />New policies:
Forward guidance.Inflation targettingFunding for lending scheme