The two things I rely on the most, are a command shell and a web browser... yeah. Over the years, these have almost become like meta-user-interfaces for me. The CLI allows me a very effective way to manipulate the file system, and while cmd sucks at everything it does, modern unix shells throw in enough to create a very powerful environment. The main cost in learning how to use the command prompt, is learning how to say what you want, rather then point at it like a child. In all my years, I've still not seen a file manager application that can match a proper unix userland and scriptable Bourne style shell. Web browsers have been a huge part of my life, since the WebTV era lol. E-mail. calendaring, task management, keeping up with the news, bookmarking, even my journal, is all web centric for me.
Both a terminal emulator and a browser, dominate my screen real estate. After that, basically comes instant messengering, which is arguably the only GUI+Desktop apps I really use all day. Other software like geeqie and mplayer fall into special use cases.
When you use software a heck of a lot, you learn to leverage it for every ounce it is worth. For example, vim is an extremely powerful editor, but if you only use the delete and arrow keys, you are wasting everyone's time. I spend enormous amounts of time with text, so efficient editing of it matters to me: why spend an hour doing what software can do in seconds? Uh huh. My love for Bourne style shells, comes from the ease of scripting: whatever I can do in a script, I can do at the prompt; making arbitrarily complex tasks accessible. I'm sorry to say that Microsoft's cmd is a pile of junk. PowerShell is improved, but still lacking compared to most unix shells \o/. Web browsers are still very unevolved creatures, I enjoy chrome because it's unobtrusive and actually tries to get out of my way; it's also proven to be an order of magnitude more stable than Netscapes bastard has been over the years.