Guilio has some valid comments about google's definition of 'related' links. He's in the hotel industry and their site links to "complementary" resources which are not hotels but these are things that their hotel guests might consider relevant in staying with their hotel.

Example.
- Car rentals
- Airport transfer services
- Bicycle Rentals
- art galleries
- Cinemas
- Museums
- Theaters
- Bars
- Food Festivals
- Restaurants
- Casinos (Yes, if you book an hotel in Las Vegas, you want to know the best casinos if you don't have one inside your hotel)
- Clubs and Discos
- Festivals & events
- Nightclubs

He is quite concerned about Matt Cutts' comment about a real estate site linking to a mortgage site which he tagged as 'low quality links' because that example might be applied to his case.

It would not be a good move for them to link to other hotels. That does not sound right on a business perspective. On a guest perspective however, it would be totally logical for this hotel site to link to things that are important to the guest such as the list above.

I totally believe that somehow, google has the technology to map these links as "related". Back in the google labs, they have this thing called 'sets'. It can suggest similar topics if you give them some 'seed' keywords.

Try hotel, car rental, restaurant, tourist spot and it will come back with 'related' topics such as apartment, gym, general retail, cruise, pro shop, club house which are quite 'related' and complementary.

If google has this, then I guess they are using this technology in their ranking algorithm.