The L-suffix parts are guaranteed (and presumably tested) to work at 8MHz when powered at 2.7V, while the non-L suffix parts are guaranteed to run at 16MHz at 4.5 to 5V. The general theory is that ALL parts will run 16MHz at 5V, but only some will run 8MHz at 2.7V, so you are likely to be able to run the L parts at 16MHz if the supply voltage is high enough, and somewhat less likely to be able to run the non-L parts at low voltages.
Counting on this in critical applications would be a bad idea.
But you'll notice that the newer parts (328) have done away with low-voltage designated parts, and just provide a curve of expected max clock rate vs supply voltage...