As well as the frameworks listed, two other prominent implementations are Concierge and ProSyst mBedded Server, although these are both oriented towards mobile and embedded use.
For server-side work an alternative to building directly on a standalone OSGi framework is to use a value-added distribution like Apache Karaf, Eclipse Virgo or Paremus Nimble (I work on Nimble).
A useful way to understand this is to think of Framework implementations like Equinox, Felix and Knopflerfish as analogous to OS kernels such as the Linux or BSD kernel, and to think of Karaf, Virgo and Nimble as analogous to OS distributions like Ubuntu and Suse. When it comes to Unix, most organizations adopt a distribution rather than a kernel, and from what I can see, the same thing seems to be happening with OSGi.
The extra facilities that Karaf, Virgo and Nimble add to a raw OSGi framework are things like:
- powerful shells
- scripting support
- ssh access
- bundle dependency resolution, provisioning and diagnostics
- pre-configured logging
- support for working with WARs
- OS integration (ability to run as a service)
- Ability to switch easily between frameworks
All of these projects are under very active development so for the latest details of each it's best to look directly at their websites.