When it comes to free - and I mean literally free - plug-ins, one of my favorite companies is Togu Audio Line.
Their filters, vocoder, reverb, and softsynths like Bassline (pictured above) and Elek7tro ooze presence and character. In fact, I'm using the TAL-Bassline on one of my newest tracks. Their Juno-60 emulation's pretty darn good too.
Available for both Mac and PC.
Here's the link. Check 'em out.