This tool is very useful for network administrators. GetSID Converts a username to a SID. It is a simple tool to use. Simply enter a username and a domain and GetSID will find out what the SID for the username is on a Microsoft domain network. This version is the first release on CNET Download.com.
Let's you specify the domain, but doesn't seem to be successful except for your current domain. No way to provide credentials if you want to query another domain.
Summary
Very much a niche tool, but if you need a simple way to get SIDs, this will do it.