The 15 seats of Yisrael Beiteinu would be extremely valuable to either Kadima or Likud in forming a coalition government. The Shas Party is hoping an alliance with United Torah Judaism, which would offer a combined 16 seats, will be even more valuable.
Both Shas and United Torah Judaism (UTJ) are Ultra-Orthodox religious parties, with the key difference being that the Shas Party represents one ethnic subgroup (the Sephardi) while the UTJ represents another (Ashkenazi). Both are considered right-wing parties, but have considerable problems with the right-wing secularist Yisrael Beiteinu party led by Avigdor Lieberman. The both took different tacks in the election, UTJ courted the Arab vote while Shas leader Rabbi Yosef just declaring voting for Lieberman an unforgivable sin, but the talks occurring today suggest the two may be close to uniting with the common cause of stopping Lieberman.
Whether they can succeed or not remains an open issue, but if they are determined to place themselves in opposition to Yisrael Beiteinu they may make it a lot more difficult for either Kadima or Likud to form a party.