I have a fun one. The front door to my house had an automatic door opener, paired with a single-button remote control to unlock and open the door. The remote control was annoying to carry and use. (This was before IoT became a thing.)

I pried open the remote, soldered on an extra circuit bypassing the push switch, and hooked it up to an Arduino. When a packet is sent over serial, the Arduino simulates a button push:

    const int basePin = 2;

    void triggerRemote() {
        digitalWrite(basePin, HIGH);
        delay(2000);
        digitalWrite(basePin, LOW);
    }

    void setup() {
        pinMode(basePin, OUTPUT);
        Serial.begin(9600);
    }

    void loop() {
        if (Serial.available() > 0) {
            Serial.read();
            triggerRemote();
        }
    }

This was paired with a tiny web server to do the serial write:

    #!/opt/bin/python2.6

    PORT = 5525

    import BaseHTTPServer, SocketServer

    class LoccaHTTPRequestHandler(BaseHTTPServer.BaseHTTPRequestHandler):
        server_version = "LoccaServer/1.0"
        def do_GET(self):
            if self.path.startswith("/trigger"):
                serial.write('A')
                self.send_response(200)
            else:
                self.send_error(404)

    serial = open("/dev/ttyACM0", 'wb', 0)

    httpd = SocketServer.TCPServer(("", PORT), LoccaHTTPRequestHandler, False)
    httpd.allow_reuse_address = True
    httpd.server_bind()
    httpd.server_activate()
    httpd.serve_forever()

Finally I threw together an iPhone app with the most basic UI imaginable: a static full-screen photo of the remote; tap once, it fires off a HTTP request, and the door swings open:

    - (IBAction)triggerRemote:(id)sender {
        NSURL *url = [NSURL URLWithString:@"http://10.0.8.48:5525/trigger"];
        NSURLRequest *request = [NSURLRequest requestWithURL:url];
        [NSURLConnection connectionWithRequest:request delegate:nil];
    }

That's basically all of the code. Considering how much of a janky hack this is, it worked great.

Ancient write-up with some photos: https://web.archive.org/web/20120103180640/http://ghughes.co...