I've become hooked on the Topaz Labs collection over the last little while. The two biggest I've put to substantial use are:
Strengths are that it's an excellent noise reduction package, especially if you follow their instructions in their tutorial videos. There are a number of reviews for this utility out there, so a quick Google search will easily find them. The Luminous Landscape folks did a review on the previous version that convinced me to try it. I tried it and I stopped using Noise Ninja (which is also excellent).
I got to admit, I'm not finding a huge amount of weakness in it. It does precisely what it claims to do and it does it very well, hard to beat, though it doesn't have the brush feature of Noise Ninja for undoing noise reduction in spots. I'm using it a little less now, and lot lighter when I do, since I moved the Pentax K-5, but I'm still using it. Here's the outcome of an ISO 20000 (not a typo) shot of mine:
Visit my website (linked in my profile) to get a larger version of that if you want. Still shrunk for the web, but about twice the size.
In terms of software, it's a Photoshop plugin and covers from PS7 and up, 32 and 64 bit. It is compatible with other software that supports the Adobe plugin model.
The plugin is primarily designed around bringing out image detail, but it does a whole bunch of stuff that I generally end up using it for more artistic outcomes.
Strengths would be in the control, it ships with some nifty presets, but then you can tweak and fiddle with the outcome as much as you like. Weaknesses, well, again not a huge amount since it does what it claims to do. Basically, same requirements and limitations as Denoise.
I just bought Topaz InFocus (today) and so I haven't had it long enough to make comments. Preliminary reviews and some sample I saw convinced me to give a go and as they're offering an introductory price of $29.99 right now and really like the other two plugins, I figured what the heck.