Research In Motion’s BlackBerry PlayBook still lags behind competing mobile platforms in number of good, available apps in its marketplace. But it does have support from Box.net, the maker of cloud-based storage for individuals, and it’s a solid app on the platform.

A free download from the BlackBerry World app market, Box.net provides a simple, elegant entryway on the PlayBook to its online storage service. While not as robust as the browser-based version, it is on par with its available apps, for example, on the iPhone and the soon-to-be departed HP TouchPad. Its file-navigation structure is the same, and it is simply an easy-in, easy-out app and solid way to access key files and data.

The importance of Box.net’s presence on BlackBerry PlayBook is not insignificant; it provides a strong, brand-name service for RIM’s tablet platform – and a nice hook into PCs that other devices have through a variety of apps and services. And not only is the app free, but Box.net’s baseline, entry-level storage offering of 5 GB is also free – meaning those getting up and running right out of the gate on the BlackBerry PlayBook can synchronize with the service immediately.

For those with a BlackBerry PlayBook, Box.net’s app is as close to a must-have as you might get.