The DS415+

The DS415+ has just been released by Synology. Sadly it can't run DSM 5.1b yet but it can still run a few of the new DSM apps that are not usually available in the package-center. The problem is that the old apps are not adjusted for this new architecture yet, however, existing x86 apps (For example, DS412+) can still run on your DS415+ with some modifications.
This is how I do it on my Windows PC for the DS415+:
1) Install 7-zip
2) Download the latest x86-package(.spk) for your package you'd like to install.
3) Open the file in Windows Explorer using 7-zip (7-zip/Open)
4) Locate the file "INFO". Right click on it, select "edit".
5) INFO is a plain textfile. Locate a line where it says something similar like this:
arch="x86 bromolow cedarview"
manipulate it to look like this:
arch="x86 bromolow cedarview avoton"
Save it and make sure it updates the archive.
6) Pick "Manual installation" in the package center and pick your modified .spk-archive.
The .spk-file is just a TAR-file btw.
This method usually works. However, sometimes DSM complains about the fileformat. Then decompress the entire .spk-file to your harddrive and manually edit the "INFO"-file with "Wordpad" (Notepad fails for me sometimes), and then create a new tar archive with 7-zip and rename it to .spk instead of .tar.
This method works on several different packages I've tried that are not yet available for the DS415+ (Plex, CloudSync, Transmission (sort of), Proxy Server etc.) It has worked for me, but it's still a bit "hackish" so you do this on your own risk.