The free webmail client RoundCube also allows you to set up a signature for outgoing messages.  First, click on "Settings" at the top right side of the page:

On the Settings page, click on the "Identities" Tab and the following screen will display:

Currently, " This email address is being protected from spambots. You need JavaScript enabled to view it. " is our only Identity.  Double click on the email address that you want to create a signature for and now you can add your signature:

Click "Save," and now all outgoing emails will have your signature!