Having the LADSPA plugins is a great addition to Kdenlive and allows for a lot more functionality within the NLE.
However, there are a few confusing aspects:
For one, it's difficult to find the right effect when you need it - if I need a compressor, I look alphabetically and find nothing between 'Comb Splitter' and 'Constant Signal Generator'. Instead it's called 'Dyson Compressor'. Similarly, equalisation cannot be found under EQ, and High Pass/ Low Pass are under GLAME Butterworth...
How about rather than Audio Correction, Audio Channels and Audio submenus, just have one 'Audio' option with effects organised according to how they work:

Dynamic Effects > Compressor, Gate, Levels, Normalise...
Spatial Effects > Balance, Mono, Pan, Spatializer, Swap Channels...
Equalisation > Multiband EQ, High Pass, Notch, Pitch Scaler...

There are various effects which are double-ups or overlap with other effects (Volume (keyframable) and Audio Levels do essentially the same thing.)
There are also many music-focused effects which may not be so useful to video editors (Flanger, Barry's Satan Maximiser, etc)
Some simply don't work with Kdenlive's workflow and they confuse newbies ('Crossfade' for example).

Is there any way we could make a special Kdenlive/Video editing collection of LADSPA plugins containing only the most useful ones, preferably with logical names? (I'm not a developer so I don't understand exactly how these things work, any explanations would be great!)

But I don't want to make assumptions on which effects other Kdenlive users find useful or not, so maybe I should just ask - which audio effects do you actually use? what do you use them for?

Here's my list:
DJ EQ - (3-band Equalisation - Hi-Mid-Lo)
if I just need to make a rough EQ adjustment and don't want to spend lots of time

Dyson Compressor - (Compress peaks to reduce dynamic range and create room to increase audio levels)
sometimes I lightly compress a channel of dialogue to ensure even levels

GLAME Butterworth Highpass - (cuts off frequencies below a certain point)
I often apply a cutoff/rolloff below 80Hz to dialogue such as from a lavelier microphone, especially if it's going over background music. What does the 'Resonance' value change?

GLAME Butterworth Lowpass - (cuts off frequencies above a certain point)
I only occasionally use this if there's consistent very high-pitched noise

Karaoke - (removes/reduces centre-panned audio, presumably in the vocal frequency range (500-4000Hz))
useful when applied on instrumental backing music - it carves out some space, in both frequency and spatial terms, for your centre-panned dialogue track. I often use this interchangeably/ in conjunction with 'Matrix Spatializer'. I prefer Audacity's 'Vocal Remover' but 'Karaoke' does the trick.

Mag's Notch Filter (removes a band of frequencies - you can set width and centre frequency)
when there's a persistent noise, hiss or hum which can be identified and removed/reduced

Matrix Spatializer -('spreads' a stereo track by removing center-panned audio, leaving intact reverb and audio unique to each channel)
I use it in in the same way as 'karaoke' - increase 'width' of a backing track to provide room for center-panned dialogue.

Multiband EQ - (15-band equalisation)
when I need to adjust EQ more exactly and in multiple points on the spectrum

SC1
in writing this post I just discovered this plugin, which looks like a more classic compressor, with threshold, ratio etc. I might switch to this from 'Dyson Compressor' then.

Balance - (determines the mix between two channels in a stereo track)
selecting one of my stereo channels (usually external mic) and muting the other (internal mic)

Pan - (pans the audio between left and right)
once 'Balance' is applied, I pan the audible channel back to the centre.

Swap Channels - (swaps L>R and R>L)
I haven't needed this yet but can see it coming in handy!

Normalise - (brings quiet clips and louder clips up to the same maximum volume)
useful to even out the dialogue levels near the end of editing.

Volume(keyframeable) - (sets audio levels of clips)
normally I have to turn down music levels (down to 5-20 or so) while someone is talking over the top, and then turn it back up when there's no dialogue. I find the keyframing extremely unreliable - if you extend a clip after applying keyframes, a clip that you had keyframed to start at 13 percent suddenly blasts out at 100. Keyframing in the timeline is also very inaccurate. So instead of using keyframing I just chop music tracks into chunks, place them on two separate audio tracks, and extend each chunk to provide an overlap:

VOICE1 -------[dialogue dialogue dialogue dialogue]------------------------------------------------------------[dialogue dialogue dialogue dialogue]-------

MUSIC1 --[during dialogue - 10 percent volume, spatializer]----------------------------------------[during dialogue - 10 percent volume, spatializer]

MUSIC2 ---------------------------------------------------------[no dialogue - 60 percent volume, no effects]----------------------------------------------------------

and then I fade in and out between the two music channels.
You can hear this fading up and down, as well as the spatializer effect applied to music throughout this video:
http://www.vimeo.com/68335652

By far my most used audio effect is not filed under audio: Fade In/Fade Out.

So that's my audio workflow, what about yours? what are the most useful audio plugins/effects for you? what are you missing?

I use the Fade In/Fade Out a lot and the Volume/Gain tool, but don't use any of the LADSPA plugins directly (I do my serious audio mixing in Ardour & Mixbus and pull in the mixed & mastered clips in from there). Going by how plugins are managed in something like Ardour, they should be arranged by category and by title, at the very least (Ardour even lets you sort by Author). The ability to search would be handy, too.