Replacing one derailleur with another type I have an older bike (between 10 and 15 years old), which has a alivio derailleur. It's an 8 speed, and I just replaced the cassette, chain and crank set. I noticed that there was a large amount of play in my derailleur, so I reckoned it's time for a new one. Will this shimano acera derailleur fit instead of a new alivio one? They are quite a bit harder to find, and I'll just go for this if it's the easiest option. 

Thanks!

EDIT:

I replaced it this afternoon and it worked perfectly; thanks for all the advice.
The derailleur you linked is compatible.

In general, Shimano shifters for 9 speeds or less for off-road series and 10 speed or less for road series are interchangeable if the mounting scheme fits. 7-speed cassette is narrower than 8-speed, but the derailleur probably has enough room for adjustment to fit the wider cassette (edit: actually, it was 7-8 so it definitely should work). 

The derailleurs for more speeds require a corresponding shifter to work correctly. Derailleurs from other brands are also not compatible without tricks with adapters, with the exception of some old SRAM units specifically marketed as Shimano compatible.