Finding a casual game of checkers is fairly easy: PlayOK (formerly Kurnik), Gaming Wonderland, Pogo, and FlyOrDie all offer live multiplayer checkers. Or, if you prefer turn-based play-- where you need not be online at the same time as your opponent-- It's Your Turn and GoldToken both offer checkers. All these sites feature at least some strong competition, they all follow tournament rules for moving and jumping, and several of them support variants of checkers, such as 10x10 international draughts. Three on the list-- GoldToken, ItsYourturn, and PlayOK-- offer three-move restriction (the style of checkers played at most tournaments), and GoldToken also offers Eleven-Man Ballot. In general, though, the vast majority of online games are go-as-you-please, and most of the competitors are just casual players.
Serious checkers players are scattered among several sites, though some play at multiple sites. Some of the distribution comes down to personal preference, but in general serious players look for strong competition, a clean interface (having the official colors helps!), responsive tech support, consistently enforced policies about fair play, friendly players, and useful features. For example, when VOG led the pack in the late 1990s and early 2000s, it featured a polished interface, a game archive, tools to detect and punish cheaters, and-- mainly thanks to these features-- a strong competition base. Furthermore, since it was a pay site, much of the abuse common to free sites (e.g. hacking, rating manipulation, lots of immature players, and of course cheating) was absent or greatly reduced. Of course, no system is foolproof, but most ACF players tend to congregate at PlayOK or GoldToken.
Here are some things you might consider when choosing where to play:
1. How much time can you devote to a game session? Some sites offer "blitz" games (each player has 5 minutes-- or less-- to complete the game), while others allow you to play a move every few days.
2. Are you looking for practice in GAYP, 3-Move, and/or 11-Man Ballot?
3. Can you tolerate immature behavior, often including copious profanity, in the chat box?
4. Do you need access to particular tools or features, such as a move list or a game archive?
5. Do you have a sparring partner or a group of partners to play with? The right people can make up for a lot of weaknesses in site design.
Personally, I play most of my games at GoldToken, because it fits my schedule and my style. Occasionally I run into a software bug, but the site administrators are very responsive, and unlike many other sites they actually listen to their users. I recommend it as a starting point, especially if you only have a few minutes a day to devote to checkers.