So, I've been playing a lot with this stuff. I live only a mile from Paine Field, so I'm always up for sitting at the end of a runway when a 744 floats overhead. I've been able to use the tools below to better time my arrivals at airports to planewatch. Anyways...thought some might be interested.
Use Flytecomm for quick-action realtime web-based queries and seeing upcoming flight plans filed:
http://www.flytecomm.com/cgi-bin/trackflight
For more advanced detailed information, use Flight Explorer Personal Edition, which is $10 a month depending on usage. Allows access to flight plans, but not as forward-based as Flytecomm.
http://www.flightexplorer.com
For really detailed stuff, I've been playing around with AirNav's Suite. Wild stuff. Kinda expensive for personal use, but in Demo mode you can get all the info you need for the most part. Ability to read messages sent to/from commercial aircraft. Pull up flight plans, etc... I've been kinda playing with all 5 of the different products they have. They are all reasonably priced, I'm trying to determine what I will actually use. some of the products are priced on a usage basis - others on a software basis. If your an aviation fanatic, AirNav suite is definitely worth a look. Both this, and flight explorer allow you to quickly and easily identify charter flights - including Charters on various airlines. For instance I noticed last night that the Seattle Supersonics had chartered a NW A320 from Sacramento to Boeing Field Seattle. As soon as it finished refueling, it bailed off for a redeye nonstop to MSP (No PAX taken apparently). Anyways, lots of random stuff.
http://www.airnavsystems.com/