Psyco - Introduction: "In short: run your existing Python software much faster, with no change in your source.
Think of Psyco as a kind of just-in-time (JIT) compiler, a little bit like Java's, that emit machine code on the fly instead of interpreting your Python program step by step. The difference is that Psyco writes several version of the same blocks (a block is a bit of a function), which are optimized by being specialized to some kinds of variables (a 'kind' can mean a type, but it is more general). The result is that your unmodified Python programs run faster.
Benefits
2x to 100x speed-ups, typically 4x, with an unmodified Python interpreter and unmodified source code, just a dynamically loadable C extension module.
Drawbacks
Psyco currently uses a lot of memory. It only runs on Intel 386-compatible processors (under any OS) right now. There are some subtle semantic differences (i.e. bugs) with the way Python works; they should not be apparent in most programs."