I was looking to get some reliable monitors before diving in the world of vst instruments and effects.
I’m looking at schOPE and Spectro (developer: schwa).
related (edit: added threads)
Recommended Freeware Vst-plugins, A collaborative community effort! 
Mastering Advice Requested, Why are my mixes always lopsided?
Stereo Imager / Placement View
Post Your Mastering Chain, An ever-evolving thread…
http://www.bluecataudio.com/Products/Product_FreqAnalyst/ is a decent freeware…
-t
I’m using the Visualizer MusicTech Edition, which is a cutdown version of their professional metering plugin. Besides RMS levels it also offers a phase meter, which was important for me.
seems to have disappeared 
Hmm… to bad. I’d maybe shared it, but it uses a serial.
Reading through Stillwell Audio Forums on schOPE, MultiInspectorFree was mentioned and compared to schOPE’s routing…
I have MultiInspectorFree, suggested from one of the threads here. I like its multiple instance detection.
I don’t think its possible in Renoise (unless I missed something) to route different tracks to different schOPE channels (one instance of schOPE), maybe this function will change in the future…
Demoed Visualizer, lagged on my computer though…
Free phase meter… windows
http://www.kvraudio.com/wiki/?id=Phase+Meter
http://www.uk-music.de/index.php?page=gonio
Free phase meter… mac/windows
http://www.soundhack.com/freeware.php