you can download BlackBerry curve 8900 apps with or without internet data plan. There are several third-party apps that can take advantage of the Wi-Fi on the BlackBerry Curve 8900. Also, you can use the Wi-Fi browser on the Curve 8900 to download applications over the air without the BlackBerry data plan. To set the Wi-Fi browser you need to go to Options > Advanced Options > Browser and set the default browser to HotSpot Browser.
But we actually really recommend getting a BlackBerry data plan. A lot of software vendors will be only offering applications via the BlackBerry App World eventually, and a BlackBerry data plan is required in order to get applications from there. You can not download apps without the internet data plan. Here is free BlackBerry Curve 8900 applications list for your reference.