Andy writes:
Another solution, which will work but not meet code, is to get
some of those 6 outlet taps which plug directly into the dual
receptacle, and clip off the external ground plug so they can
plug in. This will not require you to go into the outlet, and can
be removed if you decide to sell the place.
It does NOT provide a ground, and is therefore no safer than
the two prong outlets, but it will work without permanent
rewiring, and is very very easy to do. Many of these 6 prong
outlets have a center screw that fastens into the dual outlet
and keeps it from pulling out or falling off.

As I say, it doesn't meet code and provides no safety, but
it works exactly like the adapter, only less likely to pull out.