I second much of the above, Toontrack, Celemony, Izotope and more...
But what has really been good for me in the past year was IK Multimedia's T-RackS Vintage Compressors Bundle. I am having a wonderful and prolific time writing music these days (style like Loggins and Messina, Livingston Taylor, Jackson Browne, Johnny Cash), based around acoustic guitar and vocals. These plugins really helped me get the sound I have been looking for in a way that I really enjoy. In other words, with some other great software, I feel I need a PhD in Celemoniac or Izotopian physics to "get "er done" and so I enter my mixing sessions with trepidation.
Back in the eighties when I first began recording in the analog studios, it was so much fun and opened a whole new realm to songwriting. The rack modules and mixing board made sense to me and it was a sound we were after. I now feel that way again and am looking forward to T-RackS British Studio Series to complement the compressors, and more.
So I thought I'd add that to the mix.
Of course, none of this would be possible without Cakewalk Sonar and so a shout out to Nick at Sweetwater who recommended this to me long ago is in order. It took a lot of doing to convert this tube head from analog to digital, but my metamorphosis is now complete :)