This is a guest post from Lior Levin.

In a day and time where people are often tied down by 10+ tasks and to-dos a day, it can be hard to keep up with it all. I find it hard to believe that any true freelancer, professional or student can live without some type of GTD (getting things done) tool; it’s just a must-have for staying organized and remembering what needs to be done for the day, week, and month.

With that being said, this list of 13 GTD tools will help you get organized on any platform – whether you’re on Windows, Mac, Linux, a mobile device or right on the web. From managing tasks to creating goals to organizing a project, this list has got you covered and at a company I work for that developed a live chat customer support software we tried almost all of them internally. So here you go:

Things

A stunning task manager with powerful features and synchronization between devices. You can view all to-do items for the current day, the next day, and future days. You can also organize/group to-dos into projects and areas of responsibility. You can even delegate to-dos to your coworkers and teammates; so it is also great for working with groups.

Available for Mac, iPhone/iPod Touch and iPad.

Todoist

Todoist is an online task manager with a very simple interface and built-in calendar. You can organize your tasks into projects and even sub-projects and sub-tasks. It also includes reminders, Gmail integration, Outlook integration, plus a Chrome extension and Firefox addon.

Available for Mac and online use.

Chandler

“A notebook you can organize, back up and share.” You can separate your to-dos into now, later, and done sections so you know what’s more important and what is already completed. You can also organize your items into collections (via drag and drop) and add stars to important notes and events.

Available for Windows, Mac, and Linux.

Beeswax

“An information management system inspired by Lotus Agenda” and only for the true geeks at heart. The UI is text based and formats your tasks in an outline-type format (similar to Microsoft Word). It is very basic, yet has quite a few keyboard shortcuts (command key binding). Tasks can be organized into categories and sub-categories.

Available for Linux.

Nozbe

This is a web-based time management and project management application that is geared toward small teams. Tasks and notes can be organized into projects and you can star the ones that are the most important. You can also attach files to your notes and tasks. Nozbe even integrates with many useful apps including: Evernote, Twitter, Google Calendar, Gmail, Yahoo, and more.

Available for iPhone/iPod Touch, iPad, and online use.

ThinkingRock

A Java based application that is great for collecting thoughts, creating to-do lists, organizing tasks into projects and sub-projects, and creating “Do ASAP” actions (for those really important tasks). You can also use it to create goals, which can then be put into a “someday” or “maybe” category.

Available for Windows, Mac, Linux, iPhone, and Android.

Toodledo

Toodledo is a very popular and powerful online to-do list organizer. Your tasks can be prioritized and organized into folders and sub-tasks. You can also create notes and goals. Toodledo has numerous integrations and other features like email, SMS reminders, IM support, a Google Gadget, Twitter support, a Firefox addon, Apple dashboard widget and more.

Available for iPhone and online use.

Remember the Milk

Remember the Milk (also known as RTM) is a popular favorite for managing tasks and creating to-do lists. It is very simple, yet offers superb functionality and integrations with Gmail, Google Calendar, Twitter and more. You can also get reminders via email, SMS or IM and share you tasks with others. There are few other great to-do list apps on the web but RTM are probably the most popular one.

Available for iPhone, iPad, Android, and online use.

MyLifeOrganized

With MyLifeOrganized you can collaborate and create tasks, sub-tasks, projects, goals and even mind-maps. The interface is customizable and actually designed to run from a portable drive. One of the most unique features is the ability to enter the amount of time required for a task. You can then ask MyLifeOrganized “I have only 30 minutes now. What is the best thing I can complete from my To-Do list?” and it will let you know which items you can do.

Available for Windows, iPhone, and Android.

Doit.im

Another simple, yet stylish to-do list manager that works on just about every platform and browser. You can organize your tasks into projects and share with others. There is also a quick-add feature, which is great if you’re in a hurry. The different view modes lets you you filter your tasks by time, context, project, contact, calendar and archiver.

Available for Windows, Linux, iPhone, iPad, Android, and online use.

TimeGT

“TimeGT is a powerful life and task management software that keeps data in sync using a secure server and freeware desktop clients.” Tasks can be tagged and organized into nested projects with unlimited depth levels. You can also separate the items that you can do now from the ones that you want to do later or someday. TimeGT also offers secure synchronization and backup so that you never lose your data.

Available for Windows, Mac, Linux, and online use.

Veetosoft

Veetosoft offers 2 different programs: uTodo (“simple and fast to-do list software”) and uOrganized (“powerful task list software”). uTodo lets you tag your tasks and organize them into lists, while uOrganized lets you tag your tasks and organize them into folders. Both offer task filtering, organization via drag and drop, keyboard shortcuts, sticky notes, themes, and the ability to run the program from a portable drive.

Available for Windows.

Wunderlist

Wunderlist is very similar to Doit.im; it’s a beautifully designed task manager that is accessible from almost any device. Items can be started to show importance, organized into lists, and shared with others. You can also customize your background with 1 of 12 built-in designs. Tasks can be filtered and viewed by due date, importance, or according to whether or not it has been completed.

Available for Windows, Mac, iPhone, iPad, Android, and online use.

What have we missed? What’s your favorite GTD tool?

This post was written by Lior Levin who works for iAdvize and is also a marketing advisor to a psd to html service company that works with designers from all over the US and Europe.