Notepad Tabs goes beyond single file editing. In addition to the tabbed interface which allows you to have multiple files open, Notepad Tabs features the ability to save sets of files in groups, so you can open up several files at once. It even includes a small file browser on the main screen to make it easier to open files. The only downside is that this browser shows only .txt files, when it's often helpful to open many other formats such as .html or .cpp. (You can still open these via the standard open dialog, of course.)
Note: This link takes you to the vendor's site, where you can download the latest version of the software.