This is one I used to use a lot, and had cause to use it this morning.  An ETL process to truncate a staging table was being blocked by another process on the server, and I needed to identify exactly what command the blocking process was attempting to execute.  I found the SPID by executing sp_who2, which showed me the SPID that was blocking the truncate command (in this case SPID 54).  I then executed the following command to find out exactly what command was causing the block to happen:

DBCC inputbuffer(54)

The result set returned shows the actual T-SQL that was last executed for that SPID, which was a very large insert from a remote PostgreSQL database. 

This is a quick and useful method for tracking down issues and investigating what is happening on your server.