There are two ways that i know to log in through ssh using a different username using the command line:
ssh username@server.yourdomain
ssh server.yourdomain -l username
The last one is my choice, and i know a lot of users did like the trinity’s scene (The Matrix), when she used that line too…

Matrix reloaded's scene (nmap's screenshot)

But if you need to login in many machines with different usernames, that can be a tedious choice. So, you can use some options in your ~/.ssh/config file:
1) If you have the same username in all servers on a specific domain, you can handle that domain with the following configuration in your ~/.ssh/config:

Host *
User userdomain1

With that configuration you can login in all machines in yourdomain1 using the username you have configured. Now, if you have another username on another domain, you can add that configuration too:

Host *
User userdomain2

Respecting the rules’ rule, you can configure an exception too. If there is a host in one of the configured domains that your account is different, you can configure it too (but it needs to be the first in order):

User userserver9
Host *
User userdomain2

That’s all