Another McDSP fan here. I have the Emerald Pack myself and I agree with Steve that the ML4000 is a fantastic plug-in.
I have been demoing the Reel Tape Suite and so far I am a big fan of the Reel Tape Saturation. It behaves quite differently from the McDSP Analog Channel; it does its own thing and what it does is really impressive. It's on my to-buy list now.
Regarding Structure, I have been putting it through its paces, and I think when it comes out it will be the first sampler I will buy. I was especially impressed with Digi's decision to partner with East-West and release a limited sample library from them along with Structure. East-West, in my opinion, makes some great sounding tools.
Newer plug-ins on my list: Waves' L3-16 and MaxxVolume have recently been released separately.
The L3-16 to me is the most transparent limiter I have ever used and competes nicely with my other favourite limiters, the Massey L2007, McDSP ML4000, and Sony (now Sonnox) Oxford.
Of course a limiter is a tool and can be used to desroy your mix or improve it. I use with caution.
Waves' MaxxVolume is something I didn't get at first; I didn't really understand what its purpose was. However, after trying it on electric bass I realized there are a ton of potential applications for this thing. I need more time with it, but alas my demo is up.
Other standbys for me in the plugin world include:
Sonalksis SV-series compressor which I use for vocals (I set it so it acts as transparently as possible).
Waves SSL bundle (I almost always use the 2buss compressor on the master fader).
EMI/Chandler TG12413 limiter (which I use on just about everything but the master buss).
URS 'A10' and 'N12' graphical eqs (great for quickly discovering problem areas and eliminating them).
SoundToys Native Effects bundle for flitering effects and echo/delay.
IK Multimedia Classik Studio Reverb (my favourite non-convolution reverb).
Massey CT4 compressor and L2007 mastering limiter.