I have created an app for Mac OS X that displays trades from MtGox in realtime.
It doesn't poll but instead opens a socket (to my realtime messaging server) that is fed with updates as they happen from the MtGox websocket service, so you see trades and prices with sub-second latency.
MtGoxWatcher, OS 10.6+, Intel-only (32 and 64 bit)
Download and info: http://sneak.datavibe.net/projects/mtgoxwatcher/
Source code is up on GitHub for the hacking (linked to from the project page above).
Feature requests and suggestions are welcome!
Jeffrey Paul <email@example.com
pub 4096R/DF2A55C2 2010-10-21
Key fingerprint = 5539 AD00 DE4C 42F3 AFE1 1575 0524 43F4 DF2A 55C2