This is not strictly software or web apps, but a Moleskine may be a great addition to many people's lives. Think of it as your Outlook when you're away from the PC. In reality all a Moleskine is, is a simple notebook, or diary (many types exist), but due to it's quality production and excellent design, many of today's geekiest people use it. I use one, I know a lot of GTD followers use one and Picasso and Hemingway used one as well.
A Moleskine is awesome, because it basically gives you the freedom to create your very own task management application, which you really can take anywhere, even in your pocket. There are no limitations, frameworks, only your mind, and since all you need is the notebook and a pen, anyone can use it.
I prefer the somewhat large daily diary. I recommend this because since it is quite thick, you can use it like a notebook, but if you do need the agenda, the dates are there for your comfort. You usually get a back pocket to sore a few bits of paper and all notebooks come with cool rounded edges, so no dog ears. They usually come around $18 for pocket versions and $25 for larger ones, but you can find it much cheaper on Amazon, or in my own Moleskine roundup.
Take a look, grab a few, even if you just take notes in it it will grow on you. If you're an artist or a musician you can find sketch pad and notation Moleskines as well!
Update: The company that produces the physical Moleskin notebooks has created a couple of applications for mobile devices such as Android or iOS that brings part of the experience to the mobile world. While it is not the same thing, the apps appear to be free and may be an option if your have your smartphone with you all the time anyway.
The apps may work for you if you have a smartphone and do not mind to use it to write down your thoughts, ideas, or whatever it is you write down.
Most Moleskin owners may prefer the real deal on the other hand, as it is more comfortable to write on and does not limit you in the same way as the app may limit you.
Ghacks is a technology news blog that was founded in 2005 by Martin Brinkmann. It has since then become one of the most popular tech news sites on the Internet with five authors and regular contributions from freelance writers.