This has the best hardware for the TI. It has a phonelinc, which can make the calc to calc link as long as you want, and also sells a radiolinc, making it possible for two or more calculators to communicate up to 600 feet wirelessly. And it is on a free megaband, so no license required (If you don't know what I mean, do not worry about it).
Great site for info. Responds to common questions seen in discussion groups, and contains additional useful information about things that can be done with a calculator.
("The best site totally devoted to the TI-89" - Protoss) Includes original TI-89 BASIC programs, selective archive of the best TI-89 games and programs with reviews, links to other TI sites, and a message board.