I, along with some friends, are planning a trip to London in September of this year. Does anyone know of any reliable websites I could find cheap air/hotel packages? Thanks in advance.

I don't know what cheap means to you or where you are located but the following websites have air/hotel packages. Most will let you extend your return flight date.
www.go-today.com
www.gate1travel.com
www.virginvacations.com
www.britishirways.com

"Cheap" in my term would be a package for about 7 days that's less than $1000.

I would like a lot of responses re: cheap. In my world, $40-$50 a for a double in the U.S. is relatively 'cheap'. But I have looked a lot and it seems like the cheapest I've seen (on various websites) is £40-£50 in the U.K. That's more like $70-$80. Maybe I haven' looked in the right places. Anyway, I would like other's ideas about this.

Go Today and Virgin both have air/hotel packages in your price range. Althought I don't know your departure dates or city there is a package to London for 7 days/6 nights (+ 1 plane night) for $549 April 1 through June 10. Then you have to add taxes of $109 and air add ons for your departure city ($50 - $250) and weekend surcharge if you must fly on the weekend. So depending on your departure city that is a range of $658 to $908. You will have choices of hotels if you want to upgrade. You can extend your air/hotel dates for extra nights or extend air return dates only for usually no extra charge.

Cool thanks!!