A retail Online RPG is a game like Diablo 2 where you buy it offline (retail) then play it online via the Internet or a service like Battle.net. RTS (real-time strategy) games with online play are listed here as well. 

Reviews

Sacred Review 
Although not as known as Diablo or Dungeon Siege, Sacred belongs with them at the top of the action RPG genre. 

WarCraft 3 Review
The Russian general in War and Peace said it best, "patience and time". Although it took several years, Blizzard has constructed a masterpiece. 

Dungeon Siege Review
Does this Diablo-wannabe succeed in surpassing the hack and slash master?

Diablo Review
One of the pioneers of online gaming.

Diablo 2 Review
This hack and slash RPG is far from being a new release, but it still has an online player base that only a few games can match. 

Neverwinter Nights Review
Along with Morrowind, this RPG has brought the entire genre to new heights. With NWN, players are in control of the gaming experiences they have (player created modules covering endless themes) instead of being limited to what the game offers right out of the box.

Games

Sacred  
Simply the best Action RPG since Diablo 2. Sacred boasts a massive non-linear world that can easily devour most of your free time. Continues to improve by fixing bugs and adding new features. 

Hellgate: London - Best Retail Online RPG For 2007
Combines the action of a FPS with the depth of an RPG. This game can be bought at retail stores and then played online for free, but players can also subscribe for updates and more features. 

Blade and Sword
Combines Diablo and arcade fighting elements into a martial arts themed RPG. Playable online or via a LAN. 

Achaea - Dreams of Divine Lands
Despite having to pay for credits, this is possibly the deepest text MUD there is. Not available in stores but you will have to pay to play it fully. 

Warlords 4
Offers multiplayer support over a LAN or the Internet.

Age of Mythology
Pit your titans against those of other players. 

Diablo 2 
Simple game play that quickly satisfies a thirst for plenty of action and treasure makes Diablo a tough game to beat in the Online RPG realm. Throw in the online playability of Battle.net and you have the recipe for on of the greatest selling PC games of all-time.

Dungeon Siege 
Dungeon Siege offers a Siege Editor in order for gamers to create their own quests called Siegelets. DS also makes it possible for gamers to trade online and offline characters.

Neverwinter Nights 
Neverwinter Nights stands out from other online RPGs by offering the toolset that was used by the developers to make the game. Thus, over 2200 player created game modules have been created with more on the way.

WarCraft 3 Essentially a strategy game with RPG elements, WarCraft 3 offers a worthy online gaming experience via Battle.net. Ladders and clans are just a few aspects of that online community. Freedom Force A very innovative PC game that finally answered the dreams of many gamers by providing quality game involving comic book super heroes. However, the game's multiplayer capabilities are disappointing due to lack of support. Instead of logging into a major network devoted to the game, you have to use GameSpy to help find opponents.

Baldur's Gate 2
BG 2 multiplayer allows 6 players to play at once and each player can have several NPCs (non playing characters). The upside to playing this game online is that you are playing one of the best computer RPGs ever. The downside is that it is not played online in many places.

Vampire The Masquerade: Redemption
Older vampire RPG that fell a little short of expectations.  

Throne of Darkness
Diablo clone set in ancient Japan. Hard to get a hold of the full game these days.  

Icewind Dale 2
Another great RPG that has enjoyed offline success, but has yet to achieve similar success online.