While I am doing revision for my econs, I cannot really think of real life examples for the 3 market structure Perfect Competitions, Monopoly, Monopolistic com. This is what I can think of, do correct me if I am on the wrong track...
Egg sellers in the wet market selling eggs-Homogenenous product(all eggs are the same)
-Many buyers, many sellers
-Free entry and exit
-Infomation and mobility(sames infos and transportation/delivery)
Microsoft-Homogenenous product(Window Vista Operating System)
-High Barriers to entry
-Infomation and mobility
(For this not very sure as...there is also other compettiors of OS like Unix, SUN)
Sony Digital-Hetogenous product(camera with sony designs, different from the rest of the retailer)
-Many sellers, many buyers
-Free Entry and Exit
-Infomation and mobility(same)
Microsoft is not an example of monopoly as you have other OS ,as you mentioned like Linux, Unix. And I don't think there are high barriers to entry also. However I can't think of any other present day example of monopoly. But in the past there was De Beers which had a monopoly in the diamond market till about a decade back when a Israeli competitor(can't remember the name) entered into the scene.
Your example of monopolistic competition is correct i think. Don't know about your perfect competition example, it could be right. PC usually exists in agricultural products whose prices are fixed by the government.