What are some alternatives?
When comparing UndoClose and Tree Style Tab, you can also consider the following products
Tabs Outliner - Next Generation Session, Windows, Tabs Manager and a TooManyTabs Solution That Really Works.
Undo Close Tab - Reopens the last closed tab.
OneTab - Whenever you find yourself with too many tabs, click the OneTab icon to convert all of your tabs into a list. When you need to access the tabs again, you can either restore them individually or all at once.
MySessions - MySessions is a simple session manager.
Session Box - Using websites with multiple accounts at the same time is made easy.
Undo Closed Tabs Button - Add a toolbar button onto any toolbar to undo closed tabs