Is it possible to get the public key of a bitcoin address I do not have the private keys for with the standard client? It is possible to get it from here:
https://blockchain.info/de/q/pubkeyaddr
https://blockchain.info/q

Is it possible with the standard client? How?

I am aware that the blockchain only contains the pubkey after a tx from that address.
It is impossible to compute the public key of an address, as the address is computed from the hash of the public key. You can retrieve the public key from address with the reference client using the validateaddress RPC call (or in the debug window of Bitcoin-Qt), but that simply fetches it from the wallet, and only works if the address belongs to you.
Update: you need to use getaddressinfo now instead of validateaddress.