(I enjoyed OpenSolaris when I used it for a while in 2009, but I gave up when Oracle took it over. I'd love to know what has happened to it since then.)
To give a quick overview on the ecosystem: everything is based around illumos as the upstream repository. There are different distributions similar what you have with linux on kernel.org and distros like ubuntu, suse, rhel, slackware. Instead of just the kernel the illumos repository also includes some libs and core system tools for userspace. The illumos distributions differentiate a bit more on use case than the linux ones.
- SmartOS is designed as a cloud hypervisor. It is probably the most radical distribution in that it is a complete live system that boots from usb/network and runs in ram. With traditional zones, KVM and lx-branded zones it's a great system for virtualization.
- OmniOS is a minimalistic server distribution. It best fits for bare metal installations.
- NexentaStor is a storage appliance built on ZFS delivering unified file (NFS and SMB) and block (FC and iSCSI) storage
- OpenIndiana continuation in the spirit of the original OpenSolaris, including a full GNOME desktop environment
- tribblix gives you "retro style and modern components"
There are a few more but these should give you a good intro.
Besides a variety of community contributors there are some bigger companies involved. Joyent, a cloud provider, is doing most work on SmartOS. Nexenta has NexentaStor, OmniOS is build by OmniTI which does consulting. Delphix is a database company that mostly contributes to ZFS parts. Pluribus has a SDN product based on illumos. Lucera, financial service provider also runs on illumos and contributes.
So even without oracle there is a very healthy mix of different commercial and community contributors.
Features like advanced filesystem support (ZFS), systems tracing (dtrace), fully fledged containers (Zones), and software defined networking (Crossbow) make it a very interesting system.
New drivers are developed and many features evolve in illumos. The focus is mostly on x86 server hardware but I know folks also using openindiana on laptops.