I love Propellerhead software, you may have noticed. I've been using Reason since version 2.5, along with ReBirth RB-338. One of the biggest disappointments is that ReBirth was never properly updated to work with x64 Windows systems and Intel macs. I did find a workaround for Windows 7, but I use a Mac now, and PPC support was dropped with Lion.
So, this is where my combinator comes in. Contained in the orange box is one single Thor synth, but with a bit of creative mapping in the Combinator I've built something that gives you easy access to all the controls you will need to get filthy, squelchy 303 sounds without having to dive too deep into Thor.
I've set Thor up so that the filter uses an 18 dB/Oct slope (like the one in the 303), and the shaper provides the distortion. Thor's sequencer can be accessed by clicking "show devices" and you can use it to program patterns.
Glide is taken care of too; Using a standard sequencer all you need to do is start the next note event before the previous one finishes, or if you're using Thor's sequencer just extend the gate length.
Download it here