I am a double CS/Linguistics major, hoping to pursue a graduate degree in computational linguistics.
Best: Google Reader
One of the interfaces I use most commonly, and very much enjoy using, is Google Reader. Clean, simple and straightforward, GReader allows me to browse through my RSS feeds very quickly and see what new information is available. Besides performing its basic functions well, GReader offers other functions that make my browsing experience more interesting. The sharing functions are very smoothly integrated, making emailing items and showing them to friends a one click process. The interface also accommodates advanced users well, providing keyboard shortcuts for those who wish to take the time to learn them.
Worst: Windows Media Player 11
Windows Media Player took a turn for the worse with its newest and flashiest interface design, in a misguided attempt to take on the popularity of Apple's iTunes. The design turned the old intuition about WMP upside down. The library is obnoxious to browse, with a few artists taking up enormous amounts of screen real estate. Playlists are completely lost upon exit, unless specifically saved to disk. It's also fairly easily to accidentally create a list of songs to burn, rather than play, or vice versa. The traditional Windows File, etc. menus are also missing, making it act unlike other applications users are accustomed to using. I find it inadequate on a basic level, even without noting some of the more interesting advanced features it lacks.
Software's Sea Change
The software technology that has changed human behaviour the most in the past five years is the many flavors of smartphones. Prior to widespread mobile OS's, people were still largely tied to paper for most informational needs outside of the house. People are using their phones to look up maps, find the best restaurant in the area, play games, and buy new products. They can surf Wikipedia, study for exams, track friends, and write emails from absolutely anywhere. If before it was uncouth to be unreachable, it's now practically unheard of to ever be in a situation where you don't have access to an information resource.
Because of this, the information bar has been raised. Though being unreachable has long been a gaffe, it has now become gauche not to be in a position to gather more information. People now expect an extraordinary level of connectivity and awareness, simply because information is so easy to come by. Whether the situation is formal or otherwise, people have no patience for those who do not know. A person who has not gotten the current dose of information seemingly loses context for interactions with the people around them.
Well-designed interfaces for smartphones -- particularly the ever-popular iPhone -- have changed human behaviour considerably. Thanks to that piece of technology, people are interacting in ways that they never could before.