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Last one I got was a free weekend of Chivalry.

You can use Is There Any Deal to monitor such activity, though they rarely have false positives.
And the game was probably Mount&Blade.

There were 2 multi-player things as trial last weekend. I don't even remember the names since they didn't really interest me. Only thing I can confirm is that they were there ;-)

I believe the two free games last weekend were Chivalry: Medieval Warfare, and Dogs of War. I know I saw Dogs of War suddenly appear in my library over the weekend, and promptly vanish along with Chivalry.

I understand your frustration. It took me a long time to realise that my Natural Selection 2 was unplayable due to an apparently "exploited key". Steam should notify us immediately when changes like this happen.

the only game i'm aware of being unilaterally removed some weeks ago from people's libraries is Order of War: Challenge, for being only multiuser and dead. hopefully is the last.

Dog's of War was having a server stress test over the weekend. It was added to everybody's accounts like a free weekend.

If you categorize all your games, newly added ones will go directly into the Games category. That way, you can easily spot recently added content in your library. About games getting removed, I have no idea about how to find it out without using a gamelist log or something like that.