If you want free, a lot of people use Sketchup http://sketchup.google.com/download/
If you want cheap (I've seen them run specials for $99), the best one out there, IMO, is Alibre: http://www.alibre.com/products/hobby/
The rest is junk, IMO. You'll burn more time on junk in a few hours than you would buying a proper tool. If your time is completely worthless, you have absolutely nothing better to do, and you'll live until the Sun supernovas, then go for it.
I am not a fan of big box bloatware (Auto-Cad, Solid Works, PTC Creo / elements/ pro-E)
And Sketchup doesn't have good import/export functions. Seems to be a good CAD tool for NON-CAD kind of people doing NON-CAD kind of jobs.
I've tried TurboCAD and found it kind of cumbersome and feature lacking (years ago, maybe its better now)
I posted TurboCAD v19.1 (x32 & x64) BTW right here:http://www.sonsivri.to/forum/index.php?topic=11684.msg137983#msg137983
Alibre looks pretty awesome.. it reminds me of SpaceClaim which, I find fairly usable having migrated from Rhino.
Don't know about the plug-in support. Anybody have Alibre links?