Either just show other products that people bought who also bought that product (I see that a lot) or you can categorize the items and show them a list of other items in that category...I am assuming you are talking about a store with products since you didnt specify.
For instance, on a music site each song has a category and a sub category (i.e Main category would be Hark Rock and Sub categories might be Neo Classical or Hair Metal or Grunge or Classic, etc..)
Based on that, if someone bought a Van Halen CD (Classic Category IMO) I could also suggest Led Zepplin or Ozzy Osbourne or Fill in the Blank
If Someone Bought a Pearl Jam CD (Grunge) I could also suggest Nirvana
Of course you could just use a single Category if you want
For Instance, If you had a category called "Dental Care" and someone bought a Tooth Brush you could list out eveything that was categorized as Dental Care such as Mouth Wash, Floss, Tooth Paste etc..
If you wanted to make some extra cash you could even make a deal with a company so that only their products show up as being suggested....instead of showing all brands of Tooth Paste you wold only show Crest, for example.
I dunno, sorry if I'm on the wrong track heh.
