Cannot access my bitcoins because my wallet is still synchronising. What can I do to spend them immediately?
One solution is to export (in your privacy) the private key connected to the address on which you received your Bitcoins (or simply all private keys). This can be done using a command line tool known as pywallet.

Then, you can import this key (or all your keys) into another wallet. Browser-based wallets such as https://blockchain.info/wallet do not require syncing. Hence you can immediately spent your coins. For the bĺockchain.info wallet the procedure is described on their website.