There is a vast difference between the various types of fermented foods available. It's a topic too wide to cover in detail here, but let me just say that homemade, naturally cultured foods are almost always preferable to commercially produced products. Commercial products are often not cultured long enough to be medicinally effective and they often contain additives which diminish the therapeutic benefit.
That said, Helios makes an excellent kefir (kefir is similar to yogurt, but contains more beneficial organisms and has a more "drinkable" consistency). I know dairy is discouraged on this diet but many people who can't tolerate dairy can tolerate kefir.
Sauerkraut is also good, but make sure to get raw (heating/pasteurization kills the bacteria and makes it useless as a fermented food). You can also obtain starter cultures (www.bodyecologydiet.com) and make your own versions for far less money. Rejuvenative foods (www.rejuvenative.com) sells a product I highly recommend called "Vegi-Delite", which is a mix of cultured beets, cabbage, etc.
In all cases, you must start SLOWLY and build up over time. If you go too quickly you will likely get worse (as someone else mentioned here). It's natural to have a little gas during the transition as the good bacteria kill off the bad. It's kind of like building up slowly with a fiber supplement.
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Re: What are some examples of fermented foods?
