How do I get the balance of an account in Ethereum?

For the new release of the web3 API:

The latest version of web3 API (vers. beta 1.xx) uses promises (asynchronous, like callback). Dokumentation: web3 beta 1.xx

Hence it is a Promise and returns String for the given address in wei.

I am on Linux (openSUSE), geth 1.7.3, Rinkeby Ethereum testnet, using Meteor 1.6.1, and got it to work the following way connecting via IPC Provider to my geth node:

// serverside js file

import Web3 from 'web3';

if (typeof web3 !== 'undefined') {
  web3 = new Web3(web3.currentProvider);
} else {
  var net = require('net');
  var web3 = new Web3('/home/xxYourHomeFolderxx/.ethereum/geth.ipc', net);
};

  // set the default account
  web3.eth.defaultAccount = '0x123..............';

  web3.eth.coinbase = '0x123..............';

  web3.eth.getAccounts(function(err, acc) {
    _.each(acc, function(e) {
      web3.eth.getBalance(e, function (error, result) {
        if (!error) {
          console.log(e + ': ' + result);
        };
      });
    });
  });

On the Web:

(Not programmatic, but for completeness...) If you just want to get the balance of an account or contract, you can visit http://etherchain.org or http://etherscan.io.

From the geth, eth, pyeth consoles:

Using the Javascript API, (which is what the geth, eth and pyeth consoles use), you can get the balance of an account with the following:

web3.fromWei(eth.getBalance(eth.coinbase)); 

"web3" is the Ethereum-compatible Javascript library web3.js.

"eth" is actually a shorthand for "web3.eth" (automatically available in geth). So, really, the above should be written:

web3.fromWei(web3.eth.getBalance(web3.eth.coinbase));

"web3.eth.coinbase" is the default account for your console session. You can plug in other values for it, if you like. All account balances are open in Ethereum. Ex, if you have multiple accounts:

web3.fromWei(web3.eth.getBalance(web3.eth.accounts[0]));
web3.fromWei(web3.eth.getBalance(web3.eth.accounts[1]));
web3.fromWei(web3.eth.getBalance(web3.eth.accounts[2]));

or

web3.fromWei(web3.eth.getBalance('0x2910543af39aba0cd09dbb2d50200b3e800a63d2'));

EDIT: Here's a handy script for listing the balances of all of your accounts:

function checkAllBalances() { var i =0; eth.accounts.forEach( function(e){ console.log("  eth.accounts["+i+"]: " +  e + " \tbalance: " + web3.fromWei(eth.getBalance(e), "ether") + " ether"); i++; })}; checkAllBalances();

Inside Contracts:

Inside contracts, Solidity provides a simple way to get balances. Every address has a .balance property, which returns the value in wei. Sample contract:

contract ownerbalancereturner {

    address owner;

    function ownerbalancereturner() public {
        owner = msg.sender; 
    }

    function getOwnerBalance() constant returns (uint) {
        return owner.balance;
    }
}

From the docs, (check out the link for variations)

web3.eth.getBalance("0x407d73d8a49eeb85d32cf465507dd71d507100c1")
.then(console.log);
> "1000000000000"