The blockchain has now reached a portly 8GB in size. For those who don't know, the blockchain is the ledger of all bitcoin transactions, which contains the cryptographic proof-of-work of all transactions. All full bitcoin clients must download this file in order to be a bitcoin node on the network.
The obvious question is where will this end? However, if you're just using the bitcoin client to look after your wallet, there's no need to run the official bitcoin app which downloads the full blockchain. If you're just an end user, check out Electrum or Multibit as light bitcoin clients. Alternatively, you can use blockchain.info to read the blockchain in a browser.
It's likely that miners who have their rigs set up to be nodes are also on fast Internet connections, and so downloading this file will not be a serious problem, even if it increases in size several more times. However, home users who are not mining should think twice about avoiding the long download by using a light client as mentioned above.
The historical size of the blockchain can be seen on Blockchain Charts. Over the past two months, the growth has been approximately linear. However, expanding the chart to 'All time' reveals that the growth has actually been exponential. That growth rate raises questions of how quickly the blockchain will grow in size as bitcoin enjoys greater adoption.