All BIS BlackBerry devices running OS 4.5 and above feature the ability to support HTML email. Read this RIM KB Article for more information about HTML email handling.
If you are using OS 4.3 and below you will not have the ability to view HTML email.
If you are using BES 4.1 SP5, it is possible to configure your account to view HTML email by following these instructions.
If your device is currently not able to view HTML email as it does not meet the OS requirements as posted above, there are two products which offer variations of HTML email support:nBlackBerrySmart Email and Empower Email.
BBSmart HTML Email Viewer BBSmart HTML Email Viewer
Empower HTML Email Viewer Empower HTML Email Viewer