Generic SQL Job Scheduler for multiple RDBMS's?
What you are looking for is an Enterprise Job Scheduler.  Computer Associates has a good one called AutoSys.  There's also JAMS, OpCon, Tivoli as well as others.  There's a decent list available on wikipedia.  I've only had experience with AutoSys, and it's expensive, but VERY powerful, but it takes some work to get setup.  I don't know the cost or complexity of the other options.

These sort of schedulers allow you to schedule commands to run on just about any OS or application that the scheduler supports without using the native schedulers (so you don't need SQL Agent or anything).