user10539: I want to build an mobile application which tells you how much your bitcoins are worth in USD. I would like to know of good API that I can rely on for data. As well as historic conversion rates over time. 
Example. 
I have 5 bitcoins -> which are worth 5031 USD. 
Bonus points for other alt-currencies included in said API. 
I would like something that isn't linked to a particular service or exchange.

user1643: The Bitcoin Charts API has current USD/BTC exchange rates sourced from MtGox.com, as well as historic trade data.
There is also the Cryptsy API and Vircurex API for alternate cryptocurrencies. I'm not sure if these would fit your needs, but they are worth looking into.

user6256: I personally like what BitPay is doing.  This is tied to their service, but it uses a rather ingenious spread of data from multiple exchanges.  They use what is called the Bitcoin Best Bid (BBB).  They pull in market data from several exchanges and create a giant table of all buy and sell orders.

The BBB is calculated by simulating an auto-routing market sell order, across all exchanges, with zero commission fees. 

For additional information see the link below:
https://bitpay.com/bitcoin-exchange-rates 
The api is available here:
https://bitpay.com/api/rates
It is updated once a minute.

user10950: It will never be accurate unless you know the preferred exchange for your user, because as you can see the prices are different for each exchange.
Go to this website http://bitcoin-analytics.com, and click on "Arbitrage", you will see the difference in exchange rates from multiple sources.
Your best bet is to ask the user to choose their exchange or if they have a preferred one, and use their API, which means you'll need to use multiple APIs for most popular exchanges, like mtgox.com, bitstamp.net, btc-e.com..etc, I would also consider which geographical area you are targeting, it would help you identify who are the ones you need to have.
Good luck.

10539: <p>I want to build an mobile application which tells you how much your bitcoins are worth in USD. I would like to know of good API that I can rely on for data. As well as historic conversion rates over time. </p>

<p>Example. </p>

<p>I have 5 bitcoins -> which are worth 5031 USD. </p>

<p>Bonus points for other alt-currencies included in said API. </p>

<p>I would like something that isn't linked to a particular service or exchange.</p>
 1643: <p>The <a href="http://bitcoincharts.com/about/markets-api/" rel="nofollow">Bitcoin Charts API</a> has current USD/BTC exchange rates sourced from MtGox.com, as well as historic trade data.</p>

<p>There is also the <a href="https://www.cryptsy.com/pages/api" rel="nofollow">Cryptsy API</a> and <a href="https://vircurex.com/welcome/api?locale=en" rel="nofollow">Vircurex API</a> for alternate cryptocurrencies. I'm not sure if these would fit your needs, but they are worth looking into.</p>
 6256: <p>I personally like what BitPay is doing.  This is tied to their service, but it uses a rather ingenious spread of data from multiple exchanges.  They use what is called the Bitcoin Best Bid (BBB).  They pull in market data from several exchanges and create a giant table of all buy and sell orders.</p>

<blockquote>
  <p>The BBB is calculated by simulating an auto-routing market sell order, across all exchanges, with zero commission fees. </p>
</blockquote>

<p>For additional information see the link below:</p>

<p><a href="https://bitpay.com/bitcoin-exchange-rates" rel="nofollow">https://bitpay.com/bitcoin-exchange-rates</a> </p>

<p>The api is available here:</p>

<p><a href="https://bitpay.com/api/rates" rel="nofollow">https://bitpay.com/api/rates</a></p>

<p>It is updated once a minute.</p>
 10950: <p>It will never be accurate unless you know the preferred exchange for your user, because as you can see the prices are different for each exchange.
Go to this website <a href="http://bitcoin-analytics.com" rel="nofollow">http://bitcoin-analytics.com</a>, and click on "Arbitrage", you will see the difference in exchange rates from multiple sources.</p>

<p>Your best bet is to ask the user to choose their exchange or if they have a preferred one, and use their API, which means you'll need to use multiple APIs for most popular exchanges, like mtgox.com, bitstamp.net, btc-e.com..etc, I would also consider which geographical area you are targeting, it would help you identify who are the ones you need to have.</p>

<p>Good luck.</p>
