Ardour (http://ardour.org/) is probably the most fleshed out one, but it's still pretty bad compared to popular commercial options (Albeton, Bitwig, Reaper, FL Studio, Logic Pro).
Hey, I think it's extremely unfair to call Ardour bad, even if only by comparison with well-known commercial options. Ardour has a very specific use case and is designed to fit that use case well.