Boxelder and Milkweed bugs
I thought these were the same bug until I looked more closely. One feeds on milkweed seeds, the other on boxelder or maple seeds. I usually notice them late summer and there's still a few kicking around.
These are the milkweed bugs:
Here's a shot of one of the nymphs moulting out of its old exoskeleton:
Now here's the boxelder bugs: