Shaunna's mention of TwitterCommons prompted me to point out poliTwitter, which aggregates federal and provincial politicians' Twitter feeds. The interface is a tad clunky (the dropdown menus are awkward to use), but there's lots here to like.
Non-politicians tweeting on political issues are captured in the "community" display. You can sort by party affiliation, and the statistics are fascinating. Trend tracking via hastag, of course. And the search allows you to find your MP/MPP.