Flow (task manager)!

Take your tasks anywhere with this feature-packed application.

Basecamp is a web-based project collaboration tool that allows groups of people to share files, meet deadlines, assign tasks, and centralize feedback.

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

TickTick is your daily must-have to-do & task list to make all things done and get life well organized.

Quire is a new-generation collaborative task management tool for organizing tasks in a tree-like structure wrapped in simple and user-friendly UI.

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).

OmniFocus is a personal task manager by The Omni Group, based in GTD method.

MyLifeOrganized (MLO) is a personal task outliner and to-do list manager which will help you to organize your goals, projects and tasks into a tree.

Redbooth (formerly Teambox), the cloud-based company redefining collaboration and busines communications. Easily organize teams and projects to ensure visibility and accountabillity, enabling your team to get more accomplished.

From a simple checklist to full-blown project management, 2Do is an incredibly powerful, award winning, task manager that lets you focus on what's important.

Simple GTD application to manage your tasks and to-dos. Syncs with web server. Complete with the usual Inbox, Today, Next, Tomorrow, Scheduled and Someday "buckets" as most GTD programs have. You can also create Projects for tasks with multiple steps.