iGTD!

Laverna is a JavaScript note-taking web application with a Markdown editor and encryption support. It's built to be an open source alternative to Evernote.

Made by the company behind WordPress , Simplenote is an easy way to keep notes, lists, ideas, and more. Your notes stay updated across all your devices. No buttons to press. It just works. Your notes are synced across your devices when you create, add to or change them.

Todoist is a task manager thats useful, fast and easy to use.

TagSpaces is an open source personal data manager.It helps you organize and browse your files on every platform.

Take your tasks anywhere with this feature-packed application.

Take quick notes on Post-It(r) like sticky notes on Windows desktop, stick those notes to websites, documents & folders. Set reminder alarm to each sticky note. To avoid desktop clutter, you can organize those sticky notes inside folders. Send sticky notes to another computer over local network (LAN) or Internet, Sync sticky notes between computers using the Notezilla.Net cloud service, access your sticky notes on any smart-device such as iOS(iPhone/iPad), Android, Windows Phone, Blackberry etc. »

An easy to use, web-based to-do list. Get organized, stay motivated, and be more productive.

Task Coach is a well-structured, portable, open source todo / task manager (supporting 50 languages). It grew out of frustration that well-known task managers, such as those provided with Outlook or Lotus Notes, do not provide facilities for composite tasks. Over time, Task Coach has added effort tracking, budgeting, notes, priority adjusting, reminders, and many other features.

Any.DO helps you get things done with your friends in a simple & elegant way. It's free, simple & fun.

Producteev is an online task management tool that serves as your full Productivity platform (Task and Project Management, Files Sharing, Internal Communication and Productivity Apps Aggregation).

Leanote provides services for note and blog. You can use "note" as your personal notebook, if you want to share with friends, just publish note or notebook to the blog.

NixNote, formerly Nevernote, is an open source client for Evernote . It is written in Java so it will also run on Windows & OS-X, but the primary focus has been to try and get a usable environment for Linux.

Org-mode is for keeping notes, maintaining ToDo lists, doing project planning, and authoring with a fast and effective plain-text system.