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
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.
The astronomy.tools is a great site for visual and imaging "expectations".
On Deep Sky Archive you can browse different views of the same object with different instruments and magnification; here you won't find any planet.
As of 2020 Deep Sky Archive no longer collects user observation, but it is available Taivaanvahti observation database. On this latest observation database you can search for Category "Solar System", 'saturn' on "Free text search" and browse the images, looking for those taken with a telescope (many of the photos are captured by smartphone). For each observation, under "Technical information" section, you can read the instrument used to take that image.