These plugins are very good as mentioned previously. They are affordable, easy to install and offer lots of options.
http://www.midi-plugins.de/ The same functions are a bit harder to find in Sonar and there are more adjustments inside Franks plugins.
The original post was about loosening up heavily quantised material. I have just finished a job making a whole lot of midi files sound good for a tribute show. I switched the sounds over from TTS 1 to very good sounding instruments both external and internal. I used Session Drummer 3 to do the drums and was very happy with the results. Except for the stiffness of the quantised drum parts. (no fault of Sonar or Session Drummer of course)
Frank's Humanize plugin does a lot to help this situiation. Firstly it is possible for computer generated randomness to be very human and musical. You just need to work out some parameters and set them accordingly. This plugin is not as simple as it looks. You must download the manual which is quite good and digest what it says about this.
I elected to adjust timing and velocity randomly. (there are other things that can be randomised too) As
Butch points out both of these are necessary. I definately agree. This can apply to a whole drum track but if you want to spilt a drum part into separate tracks for drum sounds then each of these plugins can be used on these tracks for greater control.
Timing will encompass an area either side of the actual beat. I found that 7 to 8 milliseconds either side can be good. Also the plugin does not necessarily place timing equally anywhere between say -8 and = 8 milliseconds. You can also tell the plugin to put a greater percentage of timing hits tighter in, say -3 to =3 for a different feel. Also there is a
tendancy setting which allows you to specify if you want most hits ahead of the beat, or right on, or a bit after as well. This is very cool. I found settings which made the grooves absolutely swing like hell. Taking the settings too far can sound like a drummer having a bad day on too much drink!
If you get the opportunity, record a great drummer to a click and see what the variations in timing really are. Also don't forget you can also record a great drummer, extract the groove and apply that to existing stiff midi drum parts. That can be done right now inside 8.5 Producer. I have done a bit of this but it is just slower compared to inserting a Frank plugin, tweaking up and sitting back and having a good listen. I had to get through quite a lot of material quickly.
The velocity randomness often works a treat as well. I found that many kicks with Session Drummer 3 responded very well in terms of level and sound quality to velocity variations. This setting can also get overdone then some notes just start jumping out too much, but back it off to very subtle lavel varations and you are away, it just sounds so natural. Every kick level from a live drummer would not be exactly the same. (Close though for a good drummer)