Eddie, in your plugin arsenal I would recommend you have one or two of the following which you should learn forwards and backwards - spend as much time twiddling controls in these as you have done in the past with hardware processors. It's fun; approach it that way:
A. Absolutely essential list - these are to make 'repairs' on recordings which can't first be resolved with mic selection, and mic pre-amp technique. Always try to account for issues in the signal chain before A/D if you can:
Low pass and high pass filters
EQs of both Graphic type and Parametric
Dynamics processing
B. Flavoring list - this can get real long, but on the short list are reverbs and delays, and after that chorus and other modulation effects
C. 'Mastering' list - I use the word in it's more recent 'processing' type of meaning and not 'mastering' as a mastering engineer approaches an entire album project.
My 'go-to' plugins in each list:
A. I use the HP and LP filters that are native to my DAW software, as well as the parametric EQ that's native. However, I recommend two different 'suites' of plugins and use them regularly as well: rharv already gave the link to the Kjærhus series of plugins - go grab them now. Another suite of plugins that is great is the Gvst series in the original 'skin'. Don't bother with the updated 'skin' from grymmjack, the original is actually better. I like these because the controls almost always show what they are doing to the signal with some type of graphical feedback. They are great learning and teach-yourself tools.
http://www.gvst.co.uk/ The compressor, Gcomp still reigns supreme in my book for showing what various dynamics processing controls do, both with an input/output diagram as well as displaying the changed waveform. He also has an HP and LP simple filter (often this is all you need for EQ).
There still isn't a parametric EQ in there. I rarely use anything but Tracktion's built-in 4 band, but I have used posihfopit before with success:
Actually, rather than me re-writing this guy's blog - this bedroom producer blog lists the best of the freeware plugins out there and I can't disagree with his/her recommendations:
http://bedroomproducersblog.com/2011/03/...er-vst-plugins/Surf all of their lists. Have fun. Take your time to learn the controls and the effect on the sound.
-Scott