I am writing this short topic because what speaker should be used with 1Wamp is a common question:
1Wamp uses the LM386 output stage, that drives up to 1W into an 8ohm load, the 1Wamp article says: "the output stage can drive any kind of speaker, from headphones to a Marshall 2x12 cabinet."
This is a pretty generic statement, but the fact it is that the LM386 can drive pretty much everything, so it depends on what kind of system or project do you have in mind:
The Smokey Amp (that also uses an LM386 output stage) uses a small 8ohm driver unit
At home I personally use a bookshelf size hi-fi speaker that I bought at a charity shop
I have seen people driving massive size cabinets (4x12) and it sounds pretty good as well
So essentially you can try with almost everything (car stereo speakers, etc...) and it will work, don't expect levels to compete with a live drum set but for a bedroom size amp, I can tell that is pretty loud.