Tetrifast is an enhanced version of tetrinet which removes the piece delay which plagued tetrinet. It is distributed both as a binary patch to tetrinet, and as a complete binary. This evolution dramatically increases the speed of the game. Good players can get above 100 ppm (pieces per minute) with tetrifast, where it was only possible to hit about 40, previously.
You can get tetrifast from "http://www.tetrifast.net/downloads.html". Looking for a server? Try the perl server, available at "http://www.necoware.com/~deq/tetrinet/pts/", it works great with tetrifast and is relatively easy to extend. There's a Java server at "http://home19.inet.tele.dk/aaltonen/", but it is considerably more of a pain to operate.
References:
Website: http://www.tetrifast.net/