It’s one of those things I always have to Google again and again, so documenting for my own sanity.
Here’s how to run mysqldump without any locks, which works on both MyISAM and InnoDB data engines. There are no READ nor WRITE locks, which means the dump will have little to no influence on the machine (except for CPU & disk I/O for taking the back-up), but your data will also not be consistent (!!!).
This is useful for testing migrations though, where you might not need locks, but are more interested in timing back-ups or spotting other timeout-related bugs.
$ mysqldump --compress --quick --triggers --routines --lock-tables=false --single-transaction {YOUR_DATABASE_NAME}
The key parameters are --lock-tables=false (for MyISAM) and --single-transaction (for InnoDB).
If you need more options or flexibility, xtrabackup is a tool to check out.