ERC223 token standard.

ERC223 is a modification of the ERC20 token standard.

The main goals of developing ERC223 token standard were:

1. Accidentally lost tokens inside contracts: there are two different ways to transfer ERC20 tokens depending on: 1) is the receiver address a contract, or 2) a wallet address. You should call transfer to send tokens to a wallet address or call approve on token contract then transferFrom on receiver contract to send tokens to contract. Accidentally, a call of transfer function to a contract address will cause a loss of tokens inside receiver contract where tokens will never be accessibe.
2. Inability of handling incoming token transactions: an ERC20 token transaction is a call of transfer function inside the token contract. The ERC20 token contract does not notify the receiver when a transaction occurs. Also, there is no way to handle incoming token transactions on contract and no way to reject any non-supported tokens.
3. An ERC20 token transaction between a wallet address and a contract is a couple of two different transactions in fact: You should call approve on token contract and then call transferFrom on another contract when you want to deposit your tokens into it.
4. Ether transactions and token transactions behave different: one of the goals of developing ERC223 was to make token transactions similar to Ether transactions to avoid users mistakes when transferring tokens and make interaction with token transactions easier for contract developers.

ERC223 advantages.

1. Provides the possibility of avoiding accidentally lost tokens inside contracts that are not designed to work with sent tokens.
2. Allows users to send their tokens anywhere with one function transfer. There is no difference between whether the receiver is a contract or not. There's no need to learn how a token contract is working for a regular user to send tokens.
3. Allows contract developers to handle incoming token transactions.
4. ERC223 transfer to contract consumes 2 times less gas than ERC20 approve and transferFrom at receiver contract.
5. Allows to deposit tokens intor contract with a single transaction. Prevents extra blockchain bloating.
6. Makes token transactions similar to Ether transactions.

ERC223 tokens are backwards compatible with ERC20 tokens. This means that ERC223 supports every ERC20 functional and contracts or services working with ERC20 tokens will work with ERC223 tokens correctly.

ERC223 tokens should be sent by calling the transfer function on a token contract, keeping in mind there is no difference if the receiver is a contract or a wallet address. If the receiver is a wallet, the ERC223 token transfer will be same as an ERC20 transfer. If the receiver is a contract, the ERC223 token contract will try to call tokenFallback function on receiver contract. If there is no tokenFallback function on the receiver, the contract transaction will fail. tokenFallback function is analogue of fallback function for Ether transactions, which can be used to handle incoming transactions. There is a way to attach bytes _data to token transaction similar to _data attached to Ether transactions. It will pass through token contract and will be handled by tokenFallback function on receiver contract. There is also a way to call transfer function on ERC223 token contract with no data argument or using ERC20 ABI with no data on transfer function. In this case _data will be empty bytes array.

ERC223 EIP ethereum/EIPs#223 ERC20 EIP ethereum/EIPs#20

Lost tokens held by contracts

There are already a number of tokens held by token contracts themselves. These tokens will not be accessible as there is no function to withdraw them from contract. This tokens are lost.

244131 GNT in Golem contract ~ $54333: https://etherscan.io/token/Golem?a=0xa74476443119a942de498590fe1f2454d7d4ac0d

200 REP in Augur contract ~ $3292: https://etherscan.io/token/REP?a=0x48c80f1f4d53d5951e5d5438b54cba84f29f32a5

777 DGD in Digix DAO contract ~ $7500: https://etherscan.io/token/0xe0b7927c4af23765cb51314a0e0521a9645f0e2a?a=0xe0b7927c4af23765cb51314a0e0521a9645f0e2a

10150 1ST in FirstBlood contract ~ $3466: https://etherscan.io/token/FirstBlood?a=0xaf30d2a7e90d7dc361c8c4585e9bb7d2f6f15bc7

30 MLN in Melonport contract ~ $1197: https://etherscan.io/token/Melon?a=0xBEB9eF514a379B997e0798FDcC901Ee474B6D9A1+