Which synthesis tools support VHDL libraries?
I use the following tools which all have full library support:


Riviera-PRO (from at least version 2006.02 onwards)
Xilinx ISE (from at least version 10 onwards)
Quartus (from at least version 10.0 onwards; Two or more entities with the same name cannot be used, however, only packages)
Synplify (from at least version 9.0 onwards)


The only caveat is that the XST tool in the Xilinx toolchain is incapable of performing automatic file ordering on projects where entity names exist in multiple libraries (it ungraciously gets stuck looping forever trying to resolve dependencies).