Just want to give a quick shout-out to a feature that John Du Hart's been working on: debug toolbar!
This is a pretty spiffy-looking upgrade from the old $wgDebugComments mode (which exported debug log items into <!-- HTML comments --> at the end of each page). This exports the log items, and a few other bits of info such as query data, into JSON which then gets rendered as an attractive slide-out panel at the bottom of the window.
It can use some tweaking, but I think it's going to be helpful for a lot of devs -- please try it out and give John your feedback! (Warning: current version may have some HTML injection vectors, don't use on production sites.)
Enable: $wgDebugToolbar = true;
-- brion