Update: 05 Mar 08 2.0.7
Perfect for modular software environments and cheap enough too.
Building on the success of energyXT (One of the most flexible VST sequencers), version 2.0 is re-designed with a streamlined drag & drop based interface, and adds pro features like time-strecthing, pitch-shifting and REX2 groove support. Plus support for Linux and Mac OSX.
You can even run it inside another VST host as a VST plugin.