The current version of the BMW Nav IS is V29.1.
Information on how to obtain V29.1 is in this post.

The prior version of the BMW Nav OS is V28.
Information on how to obtain V28 is in this post.

An earlier version of the BMW Nav OS is V27.1.
Information on how to obtain V27.1 is in this post.

An earlier version of the BMW Nav OS is V27.
Information on how to obtain V27 is in this post.

An even prior version of the BMW Nav OS is V26.
Version 26 Download Link is in this post.

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Sources for older versions of the BMW Nav OS:

Gren.ru hosting of older Nav OS files

Pre-Version 22 Download Link - Note: this link appears to be dead. Leaving it here for future reference, in case it comes back to life

KeyCD Download Link

Where to find CloneCD

How to burn a Nav CD using a Mac