[Zope] Which method to generate IDs for objects?

Hi! I am just experimenting with ZClasses and I ran into the following problem: when I instantiate an object this object needs an unique ID. There are some ways to generate this. First of all I thought of the system time as ID, but this can make problems, even though not with high probability. Does anybody out there have a better solution, I think of an ID generation like in a sequence: object_id_001, object_id_002, ... but how can I determine the last object ID? Other (better, more simple) ideas? Juergen