I'm a fan of Brainspawn Forte. Basically, you set up scenes; strings + piano, for example, which can be any VSTi or DXi, with effects and MIDI routing options. Then, you organise scenes into songs (piano intro, strings chorus, etc), and then create a setlist which contains songs. When the time comes to play, you just use a program change (or any other midi command, configurable) to go to the next or previous scene in the set.
i love its modular structure, you can do alot of things with it and is very simple.
it could do much more, like programming your own soundgenerators and effects, but i use itonly for playing vst-i and vst effects live.
also it works great for playing "live", with less latency over with an Impulse Cab..