This is the basic model hydrometer that comes with almost every homebrew starter kit.  It is cheap and it works very well.  The only major drawback is that it is incredibly fragile.  One must be very careful when washing and sanitizing this because one slip or tap against the sink and this will crack or outright break into little pieces!  That being said, it is plenty accurate enough for any skill level of homebrewing and cheap enough that replacing a broken one doesn't hurt too badly.