Everquest (EQ1/EQ2) is the first MMORPG to hit the high levels of over a million players, out years before the now hugely popular World of Warcraft (WoW). Everquest is run by a company called Sony Online Entertainment (SOE), however the promotion of the game is nothing like the vast media that promotes WoW.
This Guide is to help you chose an Online game to suit you, if you are after graphics you will want an entirely different game to a person who prefers gameplay, hardcore grafting, or biggest population.
MANY new online games are coming out to challenge EQ and WoW, such as Lord of the Rings, The Age of Conan, Warhammer and Vanguard, the latter also run by SOE.
If you are after population, then WoW is the most populated game by some distance. However each game has 'Servers', servers are there to spread the population so that you don't have a million people in one area that would cause lag (when your comp moves slowly and so does your character). WoW is also alot easier till you get serious and want to raid (when a whole lot of people get together to kill some tough monster, being etc.) You can play WoW on a semi decent comp which is also an advantage if you have a weaker graphics card. You can buy in-game items and GOLD for REAL money. AVOID THIS AT ALL COST, you will make money as you improve your character, and alot more self rewarding.
If you are after hardcore roleplaying where you really are up against it then you want EQ1, XP(Experience, Leveling) is slow and hard, and to get anywhere you mostly need to group. It is hard, especially at higher levels, to Solo(play at your own pace by yourself) However if you want to Solo alot, you really should not consider an Online game. This is the original MMORPG and the hardliners play here, some of the most experienced players of all RP gamers are to be found on EQ1. The downside is that it is not Newbie(Starter to game) friendly as most low levels you would team with are creations made by players who have high end characters and can't be bothered teaming with people who may slow them down. EQ1 was the best, but dying out slowly, few years left yet, but no where near as populated at its prime. as in WoW you can buy PLATINUM, rather then gold, from websites for real cash, but as this is a hardcore game you will be disliked for doing it. If you do buy False gear for Real money, it's best to keep quiet about it, SOE do not agree to such actions either.
If you want graphics, the new games such as Lord of the Rings and Vanguard are better then most, as you would expect from new games. The other upside is that nearly everyone will be new to the game and you will group and make friends quickly. However these games may also have bugs and the game may be closed at regular periods to fix such things. Vanguard has already faced many bugs.Another downside is the fact you need a much more powerful computer then WoW or EQ1. Note: Lord of the Rings is based more on the book then the film so don't expect Liv Tyler to come rescue you.
If you want the 'Jack of all trades' game then EQ2 is the one for you, better graphics then EQ1 or WoW, second biggest Population next to WoW, Can solo but can find a group easy enough, can be hard and easy, and has experienced designers to deal with bugs quicker then any of the others Online Games (Original EQ1 designers aided)
EDIT: Since the original post much has happened. WoW just keeps getting more Popular and EQ Classic will be coming out soon. Age of Conan did not have the impact first thought and it seems Warhammer is drifting the same way. Vanguard was released with too many bugs and scared many away. Lotro was good if you started it but already the newbie zones are dead in population, too much grouping is needed at the start for a character who finds it hard to find a group. WoW is still number one and it seems it is going to take something special to succeed it.
Hope this helps...See you in game:-)