Basically, online lens simulators show a visual of the perspectives you can expect from a lens you select from a drop-down list and a focal length you select using a slider. Useful when you're considering purchasing a new lens or maybe when you want to determine what lens to bring to a certain photo shoot.
Below are links to two cool interactive lens simulators available online. I wish the Nikon lens simulator had a way to change the photo. In both cases, it'd be nice to be able to upload an image of your own.
For Nikon lenses:
Nikon Lens Simulator
For Tamron lenses:
Tamron Lens Simulator
If you know other lens simulators, kindly provide the links a comment