I have come to depend heavily on the Concrete Limiter. I just love what it does to sound of things when hitting 2 to 4 db gain reduction on peaks. I stopped using Waves and Sonnox limiters in favor of it. Now, since it's a Pro Channel module, it will become unusable when the inevitable time comes that I must leave my beloved Sonar in the past.
Is anyone aware of anything out there that is similar in sound and function to Concrete Limiter? 

I'll be honest and say at 2-4 db reduction, I can't tell a difference between most limiters. I use the Waves L1 as often as the Concrete, and I hear no difference.  Maybe try that?  The L1 is "old" so it is probably cheap, and it does what a limiter is supposed to do....

Out of all the Sonar offerings I found the BT Brickwall my go to. It's only $19 if your planning on porting over. 
Most of the included plug ins are very easy to find either an alternative or you can buy them in places like the JRR shop on sale. 

I have a heap of limiters in my collection, but one of the best is completely free. Try Limiter No/6 the GUI is pretty crap but it's very usable.

Which I believe this is based on, if I'm nt mistaken
http://www.tokyodawn.net/tdr-limiter6-ge/
 

Which I believe was included in the sales (half price), so if you are in no rush, it will be again.

I've got a bunch of them, including L1, L2, Sonnox, No. 6, etc. I have just never felt any of them do quite what the Concrete Limiter does to the sound. Guess I'll just have to try them all again. Oh, I've also heard good stuff about the Fab Filter and, I do not have that one.

CL just works for me.  Have not found any that are exactly like it, but I have been experimenting with several free and non-free options.  
 
The IK VTC670 - doesn't sound anything like CL, but really liking it on my busses.  
W1 compressor (free)- a decent L1 clone.  nothing fancy
Softube Drawmer S73 - This seems like a cheat since it is literally presets with amount and gain knobs, but it is awesome on busses
 
I'm still looking

Like said before Tone Boosters Barricade limiter is cheap (in price), and is also very good.
Why not try it out to see if you like it: https://www.toneboosters.com/tb_barricade_v4.html
 

I've been trying ti find a good AU alternative to the CL for a while. The function I've found hardest to recreate in anything else is the low frequency switch.

Apart from that, Waves L2 and L3 are good limiters that are easy to configure.

d16's Frontier sounds very nice indeed but perhaps lacks the fine controls and metering for an accurate mastering final limiter.

I'd been hoping for Cakewalk to release a Mac/AU version of the CL as they did with the CA-2A, which is as good an LA-2A style compressor plugin as any and better than most. Not to be I guess.