Expands upon the example that Kamal of hkeylocalmachine.com posted on. Script is majority the same, but this example would let you define not only GETs, but POSTS and transform something like a json input into a PS Object to work against. Same caveats still apply related to security, but fascinating nonetheless.

PowerShellRestEndPoint.ps1

# Reference: http://hkeylocalmachine.com/?p=518
# Create a listener on port 7000
$listener = New-Object System.Net.HttpListener
$listener.Prefixes.Add('http://+:7000/') 
$listener.Start()
'Listening ...'
 
# Run until you send a GET request to /end
while ($true) {
    $context = $listener.GetContext()
 
    # Capture the details about the request
    $request = $context.Request
    $reader = New-Object -TypeName System.IO.StreamReader -ArgumentList $request.InputStream, $request.ContentEncoding
    
    # Setup a place to deliver a response
    $response = $context.Response

    if ($request.url.PathAndQuery -match "/end$")
    {Break}
    else {
        Switch ($request.Url.PathAndQuery) {
            default {
                $message = "<HTML><body>Unsupported</body></HTML>"
                        $response.ContentType = 'text/html'
                        $response.StatusCode = 400
            }
            "/" {
                Switch ($request.HttpMethod) {
                    default {
                        $message = "<HTML><body>Unsupported Method</body></HTML>"
                        $response.ContentType = 'text/html'
                        $response.StatusCode = 400
                        }
                    GET {
                        $message = "<HTML><body>Unsupported</body></HTML>"
                        $response.ContentType = 'text/html'
                        $response.StatusCode = 400
                    }
                    POST {
                        $message = "<HTML><body>Unsupported</body></HTML>"
                        $response.ContentType = 'text/html'
                        $response.StatusCode = 400
                    }
                }
            }
            "/whateveryouwant" {
                Switch ($request.HttpMethod) {
                default {
                        $message = "<HTML><body>Unsupported Method</body></HTML>"
                        $response.ContentType = 'text/html'
                        $response.StatusCode = 400
                        }
                    GET {
                        $message = "<HTML><body>Unsupported</body></HTML>"
                        $response.ContentType = 'text/html'
                        $response.StatusCode = 400
                    }
                    POST {
                        $jsondata = $reader.readtoEnd()
            
                        # Convert from json to PSObject
                        IF (!($v = $jsondata | ConvertFrom-Json))
                        {
                            $message = '<HTML><body>Json validation failed.</body></HTML>'
                            $response.ContentType = 'text/html'
                            $response.StatusCode = 400
                        }
                        # If conversion fails, json format assumed incorrect.
                        
                        # Test Message, basically returns the json that was posted back to you.
                        $message = $v | ConvertTo-Json -Depth 10
                        $response.ContentType = 'application/json'
                    }
                }
            }
        }
        [byte[]]$buffer = [System.Text.Encoding]::UTF8.GetBytes($message)
        $response.ContentLength64 = $buffer.length
        $output = $response.OutputStream
        $output.Write($buffer, 0, $buffer.length)
        $output.Close()
    }
}
$listener.stop()
$listener.dispose()