Update: this post was written before actual votes had been counted, when exit polling suggested a tie. Since then the vote has gone more heavily to Likud, though a Unity Government remains a possibility. The Joint List now seems likely to get 14 seats however, one more than suggested below.
Despite Netanyahu's rush to declare victory, the ultimate shape of the next Israeli government is far from clear. If the exit poll indications are borne out, with the two big parties tied at 27 seats each, Netanyahu will have a better prospect of forming a coalition than Herzog, but it would still be one with a narrow majority. Though neither Likud nor the Zionist Union want it, there may be pressure to form a National Unity Government around the two big parties. In fact, there already is pressure: Israeli President Reuven Rivlin has said he will urge them to do so, and while the Israeli President is largely powerless, he is the person who decides who will get the chance to form a government.
And if the two big parties form a unity government, then the Leader of the Opposition in the Knesset will be the head of the third biggest bloc.
And for the first time in Israeli history, that will be an Israeli Arab: Ayman Oudeh of the United List. The Arab vote, though some 20% of the population in Israel proper (within the Green Line) has usually been divided among several Arab parties and the far left joint Jewish-Arab Hadash (and many don't vote).
This year they ran united under Oudeh's leadership, and are the third biggest bloc (if the exit poll forecasts don't shift) with 13 seats. Ironic given Netanyahu's last-ditch warnings about the dangers of the Arab vote.
Lisa Goldman has a profile of Oudeh and his innovative, inclusive campaign: "Ayman Oudeh has already Won Israel's Election."