sintapilgo
Excellent performance for big files (some might know Universal Viewer for this task, it's from the same dev), simple and efficient like Np++ while still having some more advanced features. Very customizable through options and plugins.
You could find very active communities about every imaginable subject, it was super useful just like Reddit, but in a typical flat view internet forum and, most important: in Portuguese.
In the last days of orkut, when all the users looking for the "social" thing moved to Facebook and Twitter, all the activity was in communities because still today there's nothing like that.
After the end, some communities just disappeared, some migrated to own solutions like php forums and some, mainly football related, moved to VK, the Russian Facebook, which also has community forums almost exactly like orkut. I believe that 95% of activity in VK discussion boards are Brazilians, Russiand don't use that feature.
Another thing that was massive in orkut - but still niche as most users didn't use this feature - was community forums. It was great to be able to read and talk about any topic using the same account, in the same website and the same layout. Worldwide, what filled this gap was obviously Reddit, but it's still English-only for most subjects. Also the format is different, Reddit is not a typical flat internet forum.