Crackberry is the best site you can get to it on your device via wapforums.crackberry.com
I use google mail / calendar / rss reader / etc and on apps domain
So visit m.google.co.uk for all the google related apps which are:
Google mobile app
Google Sync (for calendar)
Google maps
Although i have just started getting my email via the blackberry push client (email on BB is found in setup folder). Previously i was using the excellent gmail app for BB but realised at some points it was nearly 40mins delayed.
Other apps search in google and download through BB browser:
Twitterberry if you use twitter
Opera Mini as a backup browser as blackberry browser this time round is pretty good
From Blackberry AppWorld
Facebook App
Poynt -awesome app, finds anything e.g. business, gives phone / address etc
Viigo - good for RSS feeds although i do use google reader which is quicker
WeatherBug - Good free weather App that seems to works well in UK
Quickpull - Emulates a battery pull (this is a well know term in blackberry world; basically is a full reset of the software, doesn't wipe settings but it clears and frees memory and is useful to do if the BB starts running sluggish).
Vlingo - I'm trying this at the moment, it basically allows you to operate your blackberry by voice... e.g. press a button and saying "search crackberry forums" and it will do a google search for what you said. I'm figuring out whether this is a novelty app or a worthwhile app.
Just a couple of things about the 8900 you may find useful. Being a fully featured smartphone battery life is typically quite short....
Theres a bug with the pouch that hasn't been fixed in any OS versions yet, if you use the pouch it runs the battery down quicker for some reason. Don't install endless amounts of apps unless you want them, some apps are buggy with certain OS versions which cause memory leaks making the blackberry run VERY hot and the battery will run down very quickly (a BB even working hard shouldn't really get warm let alone hot as some people report, indicating the CPU is maxing out from buggy software). Make sure you close all programs that use GPS which also eats battery and its best not to let the battery fully discharge too often. Once in every 30 charge cycles is good for full discharges. Help maximise battery life. I would also program your BB to come on / turn off when you specify, helps battery life too. And after you have finished using it, before you put it down press the top right button, on top of the device, this puts the device into standby, also saving battery.
My battery will last into 3 days with fairly heavy usage but only 2 if i am listing to music, using a lot of GPS and making a lot of calls. Which compared to the mrs iphone 3g is like for like.
I would also make sure you have OS version .206 minimum installing it with BB desktop software v4.6. I personally don't rate the latest .250 OS version, there are a couple of quirks compared to the stable .206. Version .231 is better and i would try that over .250 but if you are on orange you will already have .206. Press alt + shift + h to find out on the device. Version OS 5.0 should be here by winter time which will include some nice improvements, esp. the browser.
All in, the best phone i have ever had, which tops the previous 2 blackberrys i had which were also the best phones i ever had.

Even my mrs prefers my BB to her iphone.