Zendesk - Zendesk is a beautiful, lightweight help-desk solution.

Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.

Freshdesk - Freshdesk is a cloud-based customer support software that lets you support customers through traditional channels like phone and email, social channels like Facebook and Twitter, and your own branded community

Task Coach - Task Coach is a simple open source todo manager to keep track of personal tasks and todo lists.

HelpScout - Help Scout is a simple, straightforward way to provide excellent support

Strikethru - Strikethru is a kind of to-do list notebook for managing work lists and staying updated with the important tasks.