The new BlackBerry 9900 is absolutely amazing. I love the touch screen and the keyboard - this is the best keyboard yet. A lot of people ask me what apps they should install first.
These are the apps I recommend installing right away. I use Gmail as my primary email client and Google Calendar to manage my schedule, so this list includes some necessary apps to get the most out of that:
1) Gmail - you have to navigate to google.com
and click the "more" link to find this
2) Google Sync - same as above. This will keep your calendar updated.
3) Gtalk - same as above
4) Tune-in radio (almost every radio station on the planet)
5) Flixster (buy movie tickets)
6) One touch free flashlight (more useful than you think...)
7) BlackBerry bridge (some carriers block this, but you can find it by doing a web search)
8) BlackBerry Travel (consolidates all your travel emails and itineraries)
9) My Account app for your carrier. Track your voice, data and text usage
10) Do a search for your favourite sports team, they may have an app. The Toronto Maple Leafs have a great one.
Make sure you have a good data plan before using apps like Tune-in radio, YouTube or any other apps that stream content. Use the MyAccount app to monitor your usage. Make sure you connect to Wi-Fi wherever possible to keep the data usage down.
It varies from carrier to carrier, but most include Facebook, Twitter and YouTube as part of the basic install. Mine also included Slacker Radio which is really good.
I compared the apps that I use most on my iPhone to the above and there weren't any major ones that I couldn't find for BlackBerry. You can even put your iTunes library on it if you have a media card that can hold the songs.
Hope that helps!