I use Tracktion/Waveform which has several good quality effects bundled. In addition I use the plugins below on a regular basis. All are free except as noted...
Instruments
Synth1 - Classic no-nonsense subtractive + FM synth. Compact, well laid-out, and covers all the bases.
Specialized Synths: CheeseMachine, CosmoGirl
Drumatic4 - Drum synth with numerous natural-sounding generators and highly flexible visual configuration. Not free though some older versions are.
Organ: CMorg, Phibes
Sampler: SampleTank2, IK Multimedia Sonic Synth 2 (cheap on ebay, maybe limited to 32-bit?)
Effects
ReaComp, ReaXComp (ReaPlugs pack) - Fantastic free compressor and multiband compressors (makes a great de-esser). Logical layout, visual feedback
Voxengo Marquis, Elephant - Great compressor and mastering limiter, not free
ReaEQ (ReaPlugs pack) - Visual parametric equalizer with unlimited bands
ReaDelay (ReaPlugs Pack) - Straightforward delay with damping and unlimited taps
Voxengo OldskookVerb - Straightforward reverb
Voxengo Pspace Light - Free convolution reverb!
PSP Audioware EasyVerb - Transparent and natural sounding reverb, easy to configure. Not free
PSP Audioware PianoVerb - Free distinctive resonant filter, sounds ethereal on acoustic guitar
FreeverbToo - Open source reverb with great visual interface
Angstrolooper - Super simple but inspiring long delay (up to 32 bars!) with damping
Voxengo Boogex, Voxengo Tube Amp - Fantastic free amplifier/distortion, with convolution cabinet simulations
Pecheneg Tremolo - Highly configurable free tremolo
Tremollama - Highly configurable free tremolo
Specialized effects: Minion, HyperCyclic - looping modulation
Voxengo Span - Free spectrum analyzer (also paid version)