BExR (Bitcoin Exchange Rates) allows you to get instant exchange rates between cryptocurrencies (Bitcoin and several altcoins) and many world currencies.

~ Choose between multiple exchanges.
~ See rate at multiple exchanges for a single currency.
~ See trend as the rate changes.
~ Get notified when the exchange rate passes a threshold for a given currency.
~ Share current rates with others.
~ Receive notification when the exchange rate passes set thresholds for selected currencies.
~ See conversion rate between currencies by passing through Bitcoin (buy Bitcoin and sell for another currency).
~ Home screen widget to show current exchange rate (against a selected currency) in your launcher for any supported cryptocurrency and exchange.
~ Night mode for darker background.

Exchange rate information is provided by multiple services:
~ Blockchain (https://blockchain.info)
~ Bitfinex (https://www.bitfinex.com)
~ Bitstamp (https://www.bitstamp.net)
~ Bittrex (https://bittrex.com)
~ Coinbase (https://coinbase.com)
~ Coinfloor (https://www.coinfloor.co.uk)
~ CEX.IO (https://cex.io)
~ GDAX (https://gdax.com)
~ Gemini (https://gemini.com)
~ Kraken (https://www.kraken.com)
~ OKCoin (https://www.okcoin.com)
~ Poloniex (https://www.poloniex.com)

Supported cryptocurrencies include:
~ Bitcoin [BTC] (https://bitcoin.org)
~ Bitcoin Cash [BCC/BCH] (https://www.bitcoincash.org)
~ Dash [DASH] (https://www.dash.org)
~ Ethereum [ETH] (https://ethereum.org)
~ Ethereum Classic [ETC] (https://ethereumclassic.github.io)
~ Litecoin [LTC] (https://litecoin.org)
~ Monero [XMR] (https://getmonero.org)
~ Ripple [XRP] (https://ripple.com)

Manual refresh is limited to once each minute to avoid excessive requests to the rate providers. Automatic background refresh can be configured for varying times from as short as five minutes up to three hour intervals.

If you know of another exchange which supports retrieving rates through an API please let me know. Several exchanges were supported in the past, but had to be removed:
~ BTC-e (https://btc-e.com) - shutdown
~ BTC China (https://www.btcc.com) - no longer provides rate check api
~ Gatecoin (https://gatecoin.com) - shutdown