As Jay pointed out you can be frugal, and miserable. Best shot is the Harken Ratchamatic #2625.
Is is a load-sensing ratchet block that rolls freely in both directions when not under load. When sheeted in, it is a ratchet. When released, it rolls freely.
This means that you will have ratchet help while sheeting, but during the jibe (when you want the sheet to play out easily) is runs free.
With the old blocks the ratchet would work all the time and when you wanted the sheet to play out freely during a jibe, the wheel did not turn and you had lots of friction.., and the line would often jam and you would blow the jibe.
Just Click on the hot link below and you can buy it right here on this site. We can ship the same or next day, depending on when we get the order. Harken Ratchamatic Page