MySQL 3 and 4 (and 5):
desc tablename
which is an alias for
show fields from tablename
SQL Server (from 2000) and MySQL 5:
select COLUMN_NAME from INFORMATION_SCHEMA.COLUMNS
where TABLE_NAME = 'tablename'
Completing the answer: like people below have said, in SQL Server you can also use the stored procedure sp_help
exec sp_help 'tablename'
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