Back on my old server, I had a post about this wonderful tool called mRemoteNG. The stable version was long in the number 1.72, and required the older .Net Framework 3.5 to work. For whatever the reason, the development stopped and to get that version to work properly with multiple RDP sessions, you had to edit the EXE to support large memory pages (for those that don’t know, you had to install Visual Studio and then edit the exe with the command: editbin /LARGEADDRESSAWARE <executable with path here>) .
In any case, during the summer, I noticed on twitter, that someone had taken over the development of the software and had (finally) released an updated version that supported .NET framework 4 (as well as other improvements). After using the updated version for a couple of months, I can say that the update is really useful. You just have to tweak the UI to suit your needs after the upgrade. Old connection configurations work, but old settings might not (and most likely, will not) migrate properly.
TL;DR: mRemoteNG is a good tool, got updated during summer.
In any case, below is a bunch of links.