Word games are really popular on mobile devices. Pepper.pk just released one for BlackBerry that is already a top seller on other mobile platforms called Skitrix. With this game, letters fall from the top of the screen. Your goal is to form 3-7 letter words and when you do the words disappear. The more letters in a word, the more points you get. There are also tiles with red and blue "pills" that will clear out an entire row or column and bomb tiles that clear out a row and column simultaneously, which can really come in handy when you're stuck. It's a really challenging and entertaining game that will keep you occupied for hours.
Skitrix is on sale for $1.99 as a special introductory price, and will be $2.99 regularly. You can grab it for your BlackBerry 89xx, 95xx, 9630/50, 97xx, and 9800.