There is a new plugin information page to help those looking for plugins. One of the things it needs the most is a list of recommended plugins (LADSPA, LV2 primarily). If you have some experience with LADSPA plugins that you’d like to recommend, please comment on this story, and I’ll merge responses back into the plugins page. Yes, this is not a wiki 
This is great!
I (freely) translated it in french and added to our wiki: http://wiki.bxlug.be/ProjetAudio/Plugins
Many thanks
–
Raphaël.
Good to see the first plugin that came to my mind is already there! Hail (Barry’s) Satan (Maximizer)! 
Some lesser knowns:
Calf Plugins: http://calf.sourceforge.net/
Socal’s LEET Plugins: http://code.google.com/p/leetplugins/
DSSI-VST 0.7 now with LADSPA Extensions: http://www.breakfastquay.com/
Useful information for ArdourVST and/or DSSI-VST:
http://ladspavst.linuxaudio.org/
Let’s start from the basics:
Compression:
SC4 is the best compressor around, has also a useful meter of the amount compression applied.
EQ:
“Parametric triple band with shelves” is a great general purpose EQ to be used on each track.
Reverb: Freeverb
IMHO these have to be prominent on the list, they’re not special FX, they are needed even for the most basic job.
“Parametric triple band with shelves” is malfunctioning.
Most of the times, the plugin starts with the frequencies set to 0 Hz, which can blast your ears and monitors away, don’t ask me how this works. Just do not use it and not at all recommend it, until the author officially recognizes, understands and fixes this issue.
Benjamin
TAP TubeWarmth for adding some extra punch and presence. Apply individually to each track in varying amounts for best results… Set to “Tube” for adding warmth, “Tape” for adding presence. Too much will make a track sound “crunchy”.
Comb Splitter for when you need to make a narrow (i.e. mono) sound into a wide sound. There are some other ways to do this, but this plugin is one of the more subtle.
I’ll second the votes for SC4 and Freeverb. Maybe soon we’ll see an LV2 version of JConv/Aella…
EQ:
“Parametric triple band with shelves” is very good sounding but ALWAYS set all frequency values higher than 1hz, even unused ones… otherwise you may get strange results like DC offsets or crazy noise.
Reverb:
TAP Reverberator with Ambience preset. Sounds amazing good on voices and everywhere else where you need a clean subtile reverb.
I can also recommend Tube Warmth. But be careful… on some voices/mic combination it’s good… on some it makes things worse
Compressor:
SC4 and TAP Dynamics. Can’t decide which one is better. Currently I use SC4 for most purpose.
My $.02 on LADSPAs
Compressor - SC4 as mentioned several times
Reverbs - CAPS Plate is a very authentic darkish Plate reverb great for drums, reminiscent of Steinbergs Wunderverb. Freeverb is quite bright but useful, TAP Reverberator, Confusing interface, some horrid and some useful presets
Mono to Stereo splitter - Very useful for Plugin plumbing occasionally especially VST’s (if you use them)
Barry’s Satan Maximizer - Two controls, Phenomenally Good Compression, This is a LADSPA that is amazing in it’s own right. I always have it running in some capacity in my Master Buss.
I don’t really like any LADSPA EQ’s :(. Too bad TAP 10-band is buggy, it seem the best candidate to me.
On a semi-related topic some really stable free VST’s (ArdourVST only)
SmartElectronix Ambience Reverb *won’t save preset
SmartElectronix AnalogDelay
ANWIDASOFT DX Reverb Light - VERY NICE…and Free
Dave Brown Compressor
The entire Kjaerhus Classic series
KarmaFX Deelay, Filter, 31 Band EQ, Reverb
Reapers standalone EQ
Almost all of the Voxengo Free Plugins
BoJo Flanger, LoFi, Overdrive
SimuAnalog Guitar Suite -No GUI but Great Sound
GVst Delay
The Fish Fillet Plugins - Blockfish etc. etc.
Ericsound Plugins - FreeQ -free UniQ -Demo limited to 48khz
This is a short list that covers most common uses
I think the “Fast Look Ahead Limiter” by Steve Harris also deserves a mention here. I’m not very familiar with the Barry Satan Maximizer, but the “Fast Look Ahead Limiter” certainly does the limiting/maximizing job for me.
Also I can recommend the “Parametric triple band with shelves” as others already have mentioned.
Furthermore I very frequently uses both of the SC4 compressors (mono/stereo), which can be very powerful combined with the two aforementioned plug-ins, when mastering.
The GVerb plugin from REV plugins is an awesome sounding
reverb plugin. I compared the freely available plugins
and that felt the most comfortable:
CAPS - AmpVTS, AutoWah, and Plate/Plate2x2
BLOP - 4 Pole Low-Pass Filter with Resonance
SWH - AM Pitchshifter, Smooth Decimator
Plus the aforemention Barry’s Satan Maximizer and Fast Look-ahead Limiter…
Tom Baran’s Autotalent v0.2