The SQL Server sp_spaceused stored procedure is useful for finding out a database size, unallocated space, etc. However (as far as I can tell), it does not report that information for the transaction log (and looking at database properties within SQL Server Management Studio also does not provide that information for transaction logs).
While I can easily find the physical space used by a transaction log by looking at the .ldf file, how can I find out how much of the log file is used and how much is unused?