The most useful effects (for me so far)

1. Reverb. When you are trying to do realistic instruments with freeware, its crucial to tweak them. I quickly found out that with reverb everything sounds better. The amount of reverb you need vary greatly from instrument to instrument, it can be overdone and it is a little more difficult to master with reverb. There are already different kinds of reverb plugins in LMMS. My favorite is "2x2plate" but isn't always what one need. "Calf Reverb" is the one I recommend. "TAP Reverberator" is very interesting with its comb filter, but does only fit in rare occasions.

2. Amplifier. For you new users out there. This is an effect which works like the volume. Instead of adjusting each tone volume, then set the instrument volume to 200%, then set the FX-volume to 200%, you can just decide to amplify the signal with 6. There are already amplifiers in LMMS.

3. EQ. Producing the desired tone, that tone you have in your head, isn't easy at all. When you almost have the perfect instrument you definitely won't ruin it by randomly tweaking every knob and see what it does. Sometimes adding a simple equalizer is all you need to make the tone crisper, softer, hollower, shallower or whatever you like. I like to use "C* Eq2x2" a stereo 10 band equalizer. When you are going to master "Spectrum Analyzer" also comes in handy.

4. Peak Controller. When you want more punch in your beat or have reached the stadium of mixing, the "peak controller" effect is what you need. It automatically lowers a sound because of the levels of another. http://www.youtube.com/watch?v=-wVQOVCzVrc

5. Stereophonic Matrix. The big secret. Panning is crucial to make your song alive and hear like a high quality song. For placing instruments in the 360 degree sound field around your head I often use the "stereophonic matrix". I also use it to spread sound out, make it wider.

6. Wideners. "StereoEnhancer effect" is perhaps the simplest. I consider all the different chorus effects as wideners too. Phasers pan sounds forth and back like a lfo in addition to giving the sound some kind of depth. "Calf Phaser" is the one I use the most. "Tap Stereo Echo" is the best widener after my opinion. When you activate Cross Mode and/or Haas Effect and turn the delay enough down it feels both in right and left ear, everywhere!