I bought the Evenflo top of the stairs gates and I use them throughout my house and love them. They come with hinges that are designed so you can lift the gate off and have your doorway free of the gate when you don't need it. They have really good reviews on Amazon and my experience with them has been very possitive. Most adults don't have any problems figuring out how to use them. My 2 1/2 year old knows how they work, but he doesn't have the hand strength to open the latch. You can buy them at Babies R Us locally.
It is really important that you actually buy one that is meant to be used at the top of the stairs. I found it was impossible to find a safe pressure mounted one for stairs. The problem is that the pressure mounted ones need at least one bar that crosses from one side to the other without interuption so that the pressure works to hold it into place. That bar is always at the bottom, so you can step over it. Also, the gap can't be too great or you compromise the pressure. As a result, what you get with pressure mounted ones is a narrow gate area with a bar along the bottom. It just isn't safe.
If you do go with pet gates, one word of advice: make sure you get ones with bars and not diamonds. Kids figure out pretty quickly that the diamonds give them good footholds. (My nephew taught me that one!)
Good luck!