There are various ways, depending on the quality of the HTML you need and the complexity of the rich text, as well as your tolerance for kludginess.
a) If you have fairly simply rich text and don't care a lot about the exact HTML generated or the exact fidelity, you can use one of the solutions based on the GetDocumentByURL method, which uses Domino's conversion. This won't do well with images, but will do OK with most simple rich text.
b) If you can stand a certain amount of kludginess, you can set a rich text field to save in MIME and HTML, and then extract the HTML.
c) If you need a fairly high quality HTML or might have complex rich text or might have images and attachments, and if you have any budget at all for third party products, a product such as our Midas Rich Text LSX
might work well for you.