Gmail Labs has introduced yet another gadget which allows you to manage all your tasks right inside gmail. The gadget can be enabled from your Settings > Labs. This should be the first option for you.

However it doesnt come out as actual gadget but rather a  pop up like the one you have for chat. The link for Tasks is available under Contacts.

Gmail Task List

Here is what they say :

Tasks is a way to help you keep track of what you need to do and what you’ve already done.

• ENTER creates a new task, TAB and SHIFT-TAB indent and un-indent, CTRL-UP and CTRL-DOWN let you reorder from the keyboard, and SHIFT-ENTER toggles back and forth between the detailed view for a task and the main view
• After turning Tasks on, turn on Keyboard shortcuts in “Settings > General” and then use “SHIFT-T” to create tasks from your emails – even faster than using the More Actions menu