I reviled Input Quantize when it was added.  OK.  I still do.
 
But it can be using as an ersatz data thinning process for modulation control, stepped pitch bends,etc.  Under Quantize Settings -> Change, uncheck the Only Quantize Notes box.  Type in a number of MIDI ticks in the Resolution, or some shorter fractional measure as a duration.
 
Note that any values within the span of that duration window are stacked up vertically, and the highest value in that timeframe takes precedence.

b rock

dubdisciple
The ability to bounce arpreggiator to midi notes that can be edited. Arpreggiators are great but can be stiff and monotonous. Being able to instantly render out all that is heard from pressing one note or chord opens a lot of flexible ways to edit and add variety.

Automation on arp parameters can help (when they work).  But the 'hidden gem' feature here is the potential to customize your own arpeggiator sequences.  I'll shill my own freebies again here, but strictly as examples:
 
Alesion ARP Presets  Alesion Pattern Construction Kits
 
It should be easier in Sonar X to create your own / convert to .ptn files as the basis for arpeggiator customization (without having to resurrect Project5 as a translator).  Granted, there are a lot of shipping .ptn files in the Sonar package.  I just haven't found those to be the most useful for mining the deep potential.

 
These links don't seem to work

All the links on http://www.cakewalk.com/Support/Knowledge-Base/20090612/Project5-Free-Stuff are broken. Most likely the result of recent site maintenance. In the past I have sent PMs to Ryan about other broken links I have found and he gets them fixed.

Just popping back in to thank FBB and Mudgel for posting that keybinding. Saved me the effort of looking it up and it will certainly get used... constantly. :-)

scook
All the links on http://www.cakewalk.com/Support/Knowledge-Base/20090612/Project5-Free-Stuff are broken. Most likely the result of recent site maintenance. In the past I have sent PMs to Ryan about other broken links I have found and he gets them fixed.

Is this it?
http://www.cakewalk.com/Support/Knowledge-Base/20090612/Project5-Free-Stuff
 
Edit:
Nope the links on that page are dead too. 

It is the page which refers to the broken Alesion ARP Presets  Alesion Pattern Construction Kits links among others.

Elffin
Using drum maps to control articulation switches in kontakt... works great!

More stuff I never realized. I'm so glad I started this thread, if only for selfish reasons 

Here's another feature I think is underrated: MIDI FX. They're so useful for making musically helpful wholesale changes without having to edit on a per-note or per-clip basis (in particular the velocity one).
 
The quantize one is invaluable when songwriting. i can temporarily clean up a part while the songwriting inspiratino thing is happening, then go back later and do the "real" quantization that's more subtle, uses groove quantizing, etc.
 

An oldie but a goodie - The CAL script language is as powerful as it is underappreciated - at least for the functions that still work.  Over the years I've collected, created, and modified hundreds of CAL scripts that do all kinds of specialized functions.  Many of them save me hours and hours of work.  For example, I use HUMANIZE.CAL almost daily to help "de-quantize" MIDI sequences.
 
I just wish it had been upgraded and modernized along with Sonar to support all the current features.  Imagine the things we could do today with a powerful scripting language which would give us the ability to control (i.e. customize) all the newer Sonar features, such as automations lanes, ARA/Melodyne interaction, loop manipulations, comping functions, ACT programming, command macros, and a million other user-specific workflow enhancements. 
 
A guy can dream,
Kenn

Here's a simple feature I really like, the volume control in the inspector window...so handy and it's right there for you....that's something I always wanted in Adobe's Audition but it was never there.....you had to hunt around to adjust the volume, and sometimes that would be a bad situation, if something was playing really loud. Having a volume control right there is great.