There are a couple options here, depending on how customizable you want it:

1) Use the External Content viewlet, which is essentially a managed iframe. With this approach, you craft a separate page on a server that does exactly what you want. The page should work by opening it directly in a browser. Then, you just embed that page in your dashboard using the viewlet, by entering the URL in the preferences and setting a size. This has the advantage of being very simple, but you can't have preferences or save any data in the dashboard resource. You can see an example here, where I embedded a google calendar in my dashboard, and made the External Content viewlet trimless via the menu > Appearance settings, so you don't see a border.

2) Write an OpenSocial Gadget and add it to the dashboard. So you deploy the gadget to one of your servers, then add it to the dashboard via the widget catalog (click Add Widget in the dashboard), then use the "Add OpenSocial Gadget" link in the catalog. With this approach, you can have better integration with the dashboard with things like preferences that will be remembered, dynamic sizing of content, etc.