What is a simple command line program or script to backup SQL server databases?

To backup a single database from the command line, use osql or sqlcmd.

"C:\Program Files\Microsoft SQL Server\90\Tools\Binn\osql.exe" 
    -E -Q "BACKUP DATABASE mydatabase TO DISK='C:\tmp\db.bak' WITH FORMAT"

You'll also want to read the documentation on BACKUP and RESTORE and general procedures.

I use ExpressMaint.

To backup all user databases I do for example:

C:\>ExpressMaint.exe -S (local)\sqlexpress -D ALL_USER -T DB -BU HOURS -BV 1 -B c:\backupdir\ -DS