It depends on the model, of course - you have 8, 16 and 32 GB options. My 16 GB iPhone 3G will have about 14 GB of free space, while my 32 GB iPhone 3GS will have about 28 GB of free space. This free space is not limited to applications, nor music, nor video, nor podcasts - it is common. As long as there is free space, you can install more applications or sync more media files. There is no separation of hard disk space by file type.
On the other hand, no application can take up more than 2 GB of hard disk space.
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