Gizzmo sounds a bit harsh (and he's trying not to) but I can see where he's coming from.
Your question is as well as being frequently asked, a little wide open.
I think you also asked the same question on another thread.
So: What are you looking for?
An 8 channel preamp?
An 8 Channel ADA Converter?
One which will exceed the quality of the preamps on your interface or one which will add 8 more channels for you? Or maybe you're actually looking for an 8 channel audio interface?
And finally, what's your budget?
There are lots of models to choose from. If you have a look around and find one that looks good to you, come back with it and see what others' opinions are.
Enjoy your search.