GrowlVoice. Mac App Store. To my knowledge, no demo. Using on Lion: late iMac 27" & 11" MBA Ultimate.
I've been using it for months, and wouldn't be without it. Lives in the menu bar. One tap drops down it's screen. To make a call, you tap the phone icon in that screen's lower left corner. That opens a small window-- you tap in the first few digits of the number, OR characters of the contact's name, choosing the desired target from the instantly appearing focused list. Under that number/contact field = "Call with Phone", with a drop-down of your phones tied to Google Voice. Pick whichever one you want to use for the call.
Short version: I've tried everything available (up to a few months back), and THIS is the one that does it all, intuitively. Best of all, when there have been little buggies, the dev has been quick to provide updates. Unlike VoiceMac, this app is not static. New features appear, and are invariably genuinely useful. So, money well spent in my book. Don't recall the price when I purchased, but it was $5 or under.
BTW, it's a killer for txts too!

(Yes, I earlier purchased Big Voice or Big Phone or whatever the thing is called. Short version: big waste.)
Note: To date, I've made only USA calls, so I can't speak to Canada or others.