mysqldump -A & all-mysql-dbs.sql
This will create one file on your server called all-mysql-dbs.sql and it will contain every database inside.
Just fyi, to restore individual db's via this file is kind of a pain in the ass because this file has every db in it. If you wanted seperate .sql files you could do somthing like this.
for i in `mysql -e &show databases&`;do echo &Backing Up $i&;mysqldump $i & $i.sql;done
It will give a error in the beggining saying &mysqldump: Got error: 1049: Unknown database 'Database' when selecting the database& Ignore that... i could chop off the first line i guess
