What are the shimano rear derailleur families? What are the rear derailleur families, offered by Shimano? I need to replace a Shimano Acera derailleur and am trying to decide on a replacement part.
Cross country component


XTR [M980] (10 speed) – Top of the range for cross-country mountain
bikes
Deore XT [M780] (10 speed)
SLX [M660] (9 and 10 speed), replaced Deore LX
Deore [M590] (9 speed) Entry level cross-country mountain bikes
Non-series (brakeparts, cranks and pedals)


Trekking component


Deore XT
Deore LX
Deore


Downhill/Freeride component


Saint [M820] (10 speed) – Top of the range for downhill and freeride
bikes
Zee [M640] (10 speed)
Hone [M600] (9 speed) – discontinued in 2008


Recreational mountain bikes component


Alivio [M410 and M430] (8 and 9 speed)
Acera [M360] (8 and 9 speed)
Altus [M310] (8 speed)
Tourney (6, 7, 8 speed) – Includes several different levels of
quality, and can be found on department-store bicycles


The above list was pilfered from Wikipedia.

You're probably best to go with Acera again, to ensure it's compatible. I've had one fitted to my hybrid in the last year, so they're still available.