I'm trying to buy a beginners telescope. I have a Celestron TravelScope, but would like something better. When I look at other models, many users have posted images from their viewfinders. Is there a site that collates all of these, to give an idea of what I can expect from a given scope (e.g. is Mars just a red dot, can I make out Saturn's rings, what's the detail on the moon like, etc)?
Deep sky watch has an article that covers this
The astronomy.tools is a great site for visual and imaging "expectations".
Not a website, but Stellarium allows to show the view through different telescopes / different eye pieces or the view area of different sensors or through binoculars.
It comes with a selection of fairly usual eye pieces, telescopes and sensors and barlow lenses (if any), but you can always add your own choice to each of these categories so that you can also preview exactly of what you want to compare, if you know exactly what to compare.