Ichneumon wasp of the genus Therion. She is the third I have seen in as many days. John Sankey has also recently spotted one of these fellows in Ottawa so they are probably fairly common.
This is another case where once I start looking into it, I find out this is just one of thousands of species of insects that I had only been vaguely aware of until now. Ichneumon wasps are parasitic wasps that lay their eggs in or on another insect. The females often have a very large ovipositor hanging off the tail of their abdomen that they use to drill down into wood to reach a wood boring insect. They then deposit their egg by the prey which the larvae consumes as it grows. This particular female has a very modest sting like ovipositor visible that she will use to insert her eggs into caterpillars. The larvae feeds on the host caterpillar and the adult emerges from the pupae. Since there has to be a very close relationship between the growth cycle of the host and the parasite, Ichneumon wasps are often very host specific.