I would like to create a widget of content from my RSS feed, so to put into an HTML website.
The output should be similar to what you get from the RSS widget in WordPress.
What webapp offers a good service for something like this?
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Have a look at WidgetEasy for a simple, no-frills way for creating a widget.
If you want to customize & tweak the colours, background et al, WebRSS is a good solution.
Both the sites, however don't seem to support Atom feeds - you'll have to use this web app for that.