SKStoreProductViewController lets you do this. You just set up the itunes id of the application you want to download, and then you present it as a modal view, and the user can take it from there.
You can have a presentation table with a list of applications that you want to download, each with their itunes identifier, and when you select one of the elements, you can present the view controller.
It looks something like this (iOS 6 screenshot)

Usage example:
SKStoreProductViewController *storeViewController =
[[SKStoreProductViewController alloc] init];
storeViewController.delegate = self;
NSDictionary *parameters =
@{SKStoreProductParameterITunesItemIdentifier:
[NSNumber numberWithInteger:333700869]};
[storeViewController loadProductWithParameters:parameters
completionBlock:^(BOOL result, NSError *error) {
if (result) {
[self presentViewController:storeViewController
animated:YES
completion:nil];
}
}];
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