Here is the second post on "must have apps", this time the nonfree version. You can read the "must have free apps" post with free apps here. Now, this is a must have only if you got some money to spend for some BlackBerry apps, if not, there are a lot of cool free apps too (you can find those in the previous mentioned article).

DetaViz Documents to go Premium - this app is very expensive ($69.99) but it's very useful if you use Microsoft Word very often; now you can do it everywhere

Guitar Hero World Tour Mobile - actually this is not an application, is a game and it costs only $9.99; this is probably the type of game that any touch screen phone has on it; I even made a review of it here

gWhizPro graphing calculator - this app is very useful for students and it costs only $9.99, just like the guitar hero game. It is useful in any field from biology to Math. It is available for BlackBerry OS 4.2, 4.3, 4.5, 4.6, 4.7 Non-Touch and 4.7 Touch Screen.

BlackBerry Viewer - Again, this app is not for any people. If you have something on your BlackBerry and you want to show in front of a class, you must have this app and it costs only $35 with the calculator app plus the viewer app.