Dreamhost also provides roundcube as an option. To test drive it go to https://roundcube.dreamhost.com/ and for user put your FULL email address (i.e. firstname.lastname@example.org) and the pass is your mail password.
If you then want this interface to work at webmail.domain.com instead of squirrelmail, then go to the webmail tab in the panel and click the "edit" link for your domain, and then check the "enable roundcube" box and resave. It will most likely take a few hours for the DNS change that will occur from that change to propagate.