Revo Uninstaller has a lot of great features, and the free version is something everyone should have. It's better at finding leftover files and registry entries than Windows' built-in uninstaller, and it includes things like a "Hunter Mode" that let you click on any running application to uninstall it.
The pro version is even better though: Not only does it have more advanced scanning techniques, but you can monitor system changes, back up your registry before you delete old keys and uninstall multiple programs at once.
It's usually $40, but it's currently 50 per cent off. It could be worth your while, especially if you like to try out new software a lot and therefore constantly uninstalling old apps.