For the simplest workflow, use a photo-sharing service that already includes slideshow support. Based on the answers, Flickr is an easy option: upload the images, group them into a set, and the slideshow is available right away. Picasa/Albums was also mentioned as similarly straightforward.

If you want a more video-style slideshow, Animoto was recommended as a very simple service.

If you prefer generating your own gallery and uploading it yourself, jAlbum was suggested as an easy tool: pick a style, point it to a folder, and it creates an album with slideshow capability. Lightroom can also export HTML/Flash slideshows, but that’s probably overkill unless you already use it.

So, for the least hassle, start with Flickr or a similar hosting service with built-in slideshow support.