there is this circuit which uses xor and inverters/buffers, which supposedly can double the frequency. but, as far as i know, i've not come across anyone using it.
idea is that, when this is synthesized, the '!' operator is synthesized as inverter with some propagation delay(Tpd). the Tpd is adjusted such that it's 1/4th the Tpd of XOR gate. so, when these two signals are xor'ed we get twice the frequency.
Added after 18 minutes:
This is only theoretical, if we try to synthesize using any tool, in all probabilities, it might remove the inverters.