Not all online games (or lotteries) are the same.  I've played some games that have repeatedly crashed my computer, and some very obnoxious lotteries that either end up selling your name to a spam e-mail list or think they're clever making you read through an entire page of advertising to see if you might be a winner.

The games here are usually played against the machine, but my favorite games also have other people playing at the same time (it's fun to chat while playing!)

Here are the best of the best - clean play, minimum or no spam, minimum crashing, and ads for paying casinos aren't disguised as regular links:

Fantasy sports, Wall Street sports, arcade and casino games... all free!  The almost 5 million members of Sandbox can't be wrong!

Wow - Shockmachine is FREE and lets you play some GREAT games of all varieties (from action to arcade to incredible jigsaw puzzles) online OR offline! 

Lucky Surf - Play once a day (so everyone who plays has the same chance of winning.  They give you the option to receive advertisers e-mail, and they don't sell your e-mail address!

EZ Sweeps is pretty slick. Register a bunch of sweepstakes when you sign up (from Publishers Clearing House to the more obscure).  You can check in every day and they will automatically enter you in one more of your choosing.