From an April 5 FoxNews.com article:

Possible nominees include Elena Kagan, U.S. solicitor general and former dean of Harvard Law School; Judge Diane Wood, on the 7th Circuit Court of Appeals in Chicago; Judge Merrick Garland, with the federal appeals court in Washington, D.C.; Homeland Security Secretary Janet Napolitano; and Michigan Gov. Jennifer Granholm.

As with the Sotomayor nomination, Obama's new nominee wouldn't be likely to tilt the ideological balance of the court, since Stevens is considered a liberal justice and Obama is highly unlikely to pick a conservative. But the nomination would probably have more of an impact on the operation of the court than last year's did, because Stevens is the leader of the liberal wing.

Stevens, the longest-serving member of the current court, is often credited with bringing moderate Justice Anthony Kennedy to the liberal side for close cases, and it is unlikely his replacement could have that kind of influence.