A few of those Mars landers, especially the early Soviet ones, may
not have been sterilized as thoroughly as say the Vikings were.

I would consider the Martian environment to be much more friendly to
basic Earth life than the Moon any time. Plus could the microbes in
those plastic waste bags left by the Apollo astronauts even get through
to the regolith before they would have likely expired? I would not be
surprised if those bags have remaind intact on the surface.

As an aside, I am surprised the astronauts did not take those bags
back with them, if for no other reason than to avoid contaminating
the Moon with Earth life. NASA certainly made a big deal about it
back in the Ranger days; in fact that is why the early ones in that
series failed in their missions, as they were sterilized so thoroughly
that the process damaged the probes' electronics.

There is the famous case of the microbe brought back by Apollo 12
that they thought had spent over two years aboard Surveyor 3, but
later analysis indicates that the microbe was the result of post-mission
contamination by someone examining the returned Surveyor parts.

http://en.wikipedia.org/wiki/Surveyor_3

Maybe some of the Soviet lunar landers were not decontaminated as
well as the American ones, but I still have trouble imagining any
microbes exposed the lunar conditions surviving for very long.

As for all those Venus landers, the only ones where an Earth microbe
might have had a chance are the ones which were crushed high up in
the planet's atmosphere, where things are a wee bit nicer than the
Venusian surface, but the odds seem low there, too. Most of the
planet's atmosphere is soaked in sulfuric acid.

As for all those probes still orbiting the Sun or heading off into the
galaxy, I wonder how long a group of microbes could survive on them?