Don't think I've seen these in the thread yet:
Dune BE/LE - Free version of Synapse Audio's DUNE, probably the closest freeware synth to Sylenth in terms of sound character, quality, and CPU usage
http://www.mediafire.com/download/8au32 ... UNE_LE.zipXfer Freeware - A VST version of Massive's famous dimension expander, an 8 bit waveshaper, a bitcrusher, and some other useful tools
http://www.xferrecords.com/freeware/ValhallaRoom - Probably beats the shit out of whatever reverb you currently have for the price of a new game (as good as ArtsAcoustic for 1/3rd the cost)
http://www.valhalladsp.com/valhallaroomFabFilter Saturn - Saturation/distortion/dynamics unit with crazy versatility, good for everything from beefing up a track to completely smashing it. Pricey, but worth it.
http://www.fabfilter.com/products/saturnXfer LFOTool - Sidechain effects, trance gates, filtering, phasing, flanging, panning, etc. Very useful and not too expensive
http://www.xferrecords.com/products/lfo-toolTB Isone - Simulates speakers in a room, indispensable for mixing on headphones or simply checking a mix in various conditions. Very inexpensive for the quality and versatility it offers
http://www.toneboosters.com/tb-isone/