Using a Scarlett 6i6 USB interface and Sonar X3 Producer. I'm starting to record vocals on a project I've been working on musically for awhile now, and using the "zero latency" preset on the 6i6. However, I'm much more comfortable singing while hearing a little reverb or chorus, which seems unattainable with zero latency - just hearing the totally dry vocal. I'm wondering if there's any way to get this working through Sonar while still using that "zero latency" preset. Once it comes to "sends" and routing, I start getting a little confused. Any help or thoughts on this appreciated. Thanks.

Focusrite includes some DSP effects, including reverb, with the 6i6.
I don't know if you can send the effects to the monitor mix without recording them.
I use an external mixer to monitor, with it's reverb on. That doesn't solve your dillema, though.
Ask in the X-series forum. This forum is for older Cakewalk programs, SONAR 8.5 and below.

Thanks, Greg, will check there. Whoops!

Short answer is no, you can't use computer-based effects in real time. That's why some audio interfaces now include built-in effects that you can either record or just monitor while recording a dry vocal. But if you don't want to buy a new interface just for that, your best bet is to look around for a cheap effects unit on eBay or Craigslist. Most interfaces offer an effects loop, so there's a good chance your current interface will let you plug in an external reverb, delay or chorus with minimal hassle.
 
Because the effects in your DAW are usually better than any cheap hardware box, the best thing is to listen to the effects in your headphones while singing but record the vocal dry and add effects later. If you do it that way, you don't have to worry about the quality of the effects unit. You can find some very inexpensive units second-hand, often less for than the cost of a plugin.

Forcusrite Scarlett interfaces come with a plug in suite that includes reverb. They are not DSP. 
 
You won't get low enough RTL to use them in any musical way. There will be latency.  So best solution is to add outboard reverb with a mixer. The 6i6 has lots of options via the mix control software for cue mixes. I use a mixer so have not tried adding a hardware reverb using it's in/outs on the back, but I do believe it would work. 
 
 

I have a standard response to this question. I also have Focusrite audio interfaces and really think they are great.

But for me, for now, I use a Line 6 KB-37 when I'm tracking (recording) vocals and guitars. With the KB-37 I can really get a zero latency experience, where I have input monitoring on, and effects are fully applied.

The signal records dry, but my ears hear the effected signal. Anything from reverb on vocals to a fully effected amp sim (not even a Line 6 / POD amp sim. I use Amplitube).

So in answer to your question, yes it can be done and I do it this way. KB-37s are not expensive, and offer a lot of capability. In Sonar I switch between the KB-37 as my audio interface (for recording vocals/guitars) and the Focusrite Scarlett for everything else, especially for mixing with the Scarlett. Switching interfaces is very easy in Sonar.

Here are two links with more about the KB-37:

http://www.youtube.com/watch?v=cRpCXWSC8TE

http://www.youtube.com/watch?v=zwBR1OfHRtM

Actually in Sonar, I'm able to monitor with FX and get the latency down pretty low.  However, my Steinberg UR44 has built in DSP FX with their RevX reverb and mini channel strip.  This was mentioned above as a solution to use a card with these types of features.   Additionally, the FX are available as VST3 and my little ilock has a groovy light in it as well :-).
I like the suggestions of outboard cue systems with their own dedicated FX.  Keeps things tidy and you can get stuff for that dirt cheap nowadays.  Don't throw away all your 80's rack gear just yet.