Opera

Opera can be set to launch Eudora when you click a "mailto" link.

Open the "Tools" menu, and go to Preferences > Advanced > Programs. Double-click the "mailto" protocol and select "Open with other application". Use the "Choose" button to point Opera to the location of "Eudora.exe".

Add this as a command line argument behind the path to "Eudora.exe":

mailto:%t?subject=%s&body=%m&cc=%c&bcc=%b

This will insert the To, Subject, Body, CC (Carbon Copy) and BCC (Blind Carbon Copy) information into the e-mail you want to compose.

The resulting command line in the "Open with other application" field should look something like this:

C:\Program Files\Eudora\Eudora.exe mailto:%t?subject=%s&body=%m&cc=%c&bcc=%b