I spend a good deal of my time on brettspielwelt.de, which is a German portal for a lot of more or less popular and well-known boardgames. You might have heard of Settlers, Carcassonne, San Juan, Go, Bohnanza, etc. There's also definitely a lot of games you've never heard of. Brettspielwelt is a great way to learn and try out new games, and to play more of your favourite games when your friends are busy elsewhere. There is a handful of games in my collection bought after learning them on BSW, and another handful on the list for buying.
In addition to offering a whole lot of games, BSW is organized in a confusing mass of towns, houses and rooms. You don't need to pay attention to that if you don't want to. If you are interested, it's possible to join a town as its inhabitant (you have towns like EnglishTown, CanadaTown, ParisTown, and more general ones like Concordia (English-speaking in general), Versailles and more). On a certain level (you get a ranking after how many games you play) you can build your own house and put a game in it.
There are two options if you want to play. You can use the online web client, or download a client to your computer. The latter one gives you the opportunity to have your own settings on menus, looks and so on. Besides from the graphic user interface, you have a lot of useful and useless commands applying to help you navigate in this place. There's an overview on http://brettspielwelt.de/Hilfe/Kommandos/.
To people who'd like to try this, I have two recommendations.
1. Register. On the webpage, you can register with a username and password that you use to log in. Please do! Many users, myself included, refuse to play with unregistered users. The main reason is that some people leave the games when it becomes obvious that they will lose. If a registered user do that, he or she will get reported, and people will know to avoid that player later. All your games will also be registered, and it's easy to see if other players are at your level by their number of games and percentage of won games.
2. Be polite. It's polite to ask before seating in a game. Right-click to join a room from the game manager without joining the game automatically. Ask if you can join the game. Wish your opponents fun and good luck, and thank for the game afterwards. If the game is new to you, nicely ask a person to teach you, don't join a game pretending you know it. Also, then you should read the rules on the BSW website before playing.
All of this information, and everything else you'd like to know about this place, can be found on www.brettspielwelt.de. There are options for different languages in the upper right corner. Have fun, and good luck!