The most obvious one would be Google Gmail because DreamHost already provides an option to have Gmail handle your email in the Domains -> Manage Domains panel. In other words you login to Gmail to read and send the messages and messages sent to your addresses are sent directly to Gmail instead of DreamHost.
If you want to keep the e-mail hosted with DreamHost you can install applications like Roundcube yourself as well. Gmail also allows you to retrieve messages from your DreamHost e-mail accounts using POP.
Customer since 2000 openvein.org