The ps2 emulator PCSX2 has achieved a big speed-up making quite a few games actually playable. According to the authors on their website they achieved it through a complete rewrite of the emulator that was started with the goal to increase speed, whilst at the same time improve, or at least maintain current compatibility.

This rewrite has been going on for sometime now, and is starting to truly show fascinating results, many games now run up to 4x faster, an example of this would be Sonic Riders - previous betas ran at 3.x to 12.x FPS whilst racing, it now runs at 12.x to 35.x FPS

Testing and bug chasing continues, meanwhile a thread has been created for testers and developers to demonstrate - in screenshot form - some of these speed increases.