Hello, i'm maybe first TREO 270 user in Czech Republic (i like this PDA/phone!) and here is list of my installed colourfull applications, that works great:

Docs To Go
mMail+
33 Knots
3nity
Action Names
BDicty
BugMe!
FastWriter
Fax
LifeBalance
Lingea Lexicon
MDrink's
MobiPocket Reader
Planetarium
Ringo
Route
SplashPhoto
SplashMoney
SplashShopper
StopWatch
WAPman
WorldMate
FingerDial
Filez
MaTreo
Button-T
Treo Tools
OnCradleOn
Super Utility

and also many games... (Alchemy, Atomica, Bang!, Bejeweled!, Billiards, 300Bowl, Bubbles, Callisto, ChessGenius, Clickomania, Coinz, Hangman, Hexamine, MondoPondo, SexyPoker, SpyHunter, T-Blaster, Tennis Addict, Volleyball, ZIOGolf, etc...)

I'm interested what appz is prefered for other Treo users! Thanks!