As mentioned, you can use any of those sites. But, those are only for amateur spotters!
If you're a REAL spotter, you will have your own Air Band Reciever (scanner) which allows you to control which freq you go on, and it is very portable compared to a computer. BTW, could you just see someone pull up at a spotting place at an airport with their computer!?!?!?
"If it aint boeing, it aint going!", "Friends are like condoms...they protect you when things get hard!"