It's rare that I use plugins but got to say once in a blue moon using them can help to rescue a mix. And I've found that very often ones that are incredibly cheap can offer quite a bit of quality especially in comparison to better known and more expesive options as long as you search hard and wide enough. I find often with plugins the independent developer with no marketing dollars but plenty of coding smarts easily beats out the results given by the offerings of bigger corporate companies.
As far as specific ones that fall in this category:
Digital Fishphones Spitfish - excellent freeware de-esser
Stillwell Audio Transient Monster - similar to the SPL Transient Designer - very simple controls - but works surprisingly well across the 2-bus for adding missing transients. This one got used extensively on a recent archival reissue for a hip-hop release that had originally been recorded on 8-track cassette - putting this on really helped to tighten up some softish kicks.
Stillwell Audio Event Horizon+ - a fairly dirty sounding peak limiter/soft clipper that for some tracks can retain some nice midrange snap, and not change the bass end as much as some other limiters do, and can get tracks that already distorted into the "loud" range with less artifacts than some other choces. Not a go-to but a decent option to have that works as the best option among my other tools on say about 3% of the tracks I get here.
Acon Digital Declicker - excellent for removing clicks, ticks, and pops - works better than some more expensive plugs I've tried in fact
JMS Audioware SAWStudio Freeverb - once in a blue moon this freeware reverb plug works great for quickly automating a repair to a noisy fade or a cutoff tail.
Izotope Vinyl - once in a blue moon I get requests to make things sound "lo-fi" (usually just for an intro section) and this handy dandy piece of freeware actually can do the trick really well and very quickly.
a few freeware utilities I really like also:
Exact Audio Copy - simply one of the best freeware tools around for secure CD digital audio extraction and full featured burning from cue sheets.
Rail Rogut sdTwoWav - freeware sd2 to wav converter
Rail Rogut Interleaver - freeware dual mono to stereo interleaved batch converter
Best regards,
Steve Berson