UUID/GUID information

Dear List,
    I'm trying to migrate my MS-SQL(shit) to Postgre.  My database
depends on having a uniqueidentifier for all objects stored. (20 or so
tables of these unique objects).  In MS-SQL I can use this datatype called
"uniqueidentifier" to accomplish this.  What would be a similar solution in
Postgre?  I've looked on through the MAN pages and also scoured the net for
this info...I don't necessarly need a UUID like the MS one but some unique
way to identifiy each object.

Thanks in advance for help

/B