On a dedicated database server, how much memory to reserve for the OS?
Assuming Windows and SQL Server...

There are two schools of thought.  


Leave 2-4 Gigs for Windows (depending on what is installed besides SQL Server).
Leave 10% of your available memory free.  As you get over 64 Gigs this gets to be a crazy large amount of memory to leave for the OS, which it probably won't need.


Personally I'm in the first group.  Windows usually just needs 2-4 gigs, sometimes up to 6.