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 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.