The National Conference of State Legislatures has a recently-updated list of states that are furloughing their employees or are considering it. You can view the list by clicking here. The numbers are a bit squishy, but the chart illustrates the trend, nonetheless.
Worth noting: Alabama may soon join a growing list of states that furlough their civil servants, according to this story by Birmingham News reporter David White. It's worth noting because the proposal differs from California's furlough policy by giving state agency heads the power to assign unpaid days off to their employees.
The Alabama State Employees Association thinks the furlough talk is premature.