The two states that split their electoral votes are NE and ME. It is actually not a smart idea for most states, since that makes the state even less attractive to campaign in (and generally states like having candidates pay attention to them). Splitting the electoral votes basically transforms a larger state into a number of smaller states, from the standpoint of political campaigns. Of course, these two states happen to be small to start with (5 electoral votes and 3 electoral votes) so they're not really losing very much, and, of course, proportionally allocating the votes is "fairer".
BTW, I think calling the allocation of electoral votes in ME and NE "proportional" is actually not correct. I believe that each Congressional District elects one elector, based on who wins the popular vote in that district (that's three of the electoral votes) and then the two electoral votes attributable to the Senators are at-large, i.e., they go to whichever candidate wins the popular vote for the whole state.
I just want to emphasize something: Electoral votes allocated to state are not proportional to the state's population. For the reason I mentioned earlier, smaller states get more electoral representation than larger states.