The Longest Journey. The Feeble Files. Discworld Noir. Rent-A-Hero.
You say these titles don't ring a bell? Chances are you live in the United States, then, as the above adventure games were only released in Europe.
Don't be that shocked. The current market for adventure games is better in Europe than it is in the US. A number of European game companies still make adventures, whereas most US companies have abandoned the genre.
(A side note: this wasn't always the case. For a time adventure games were much more popular in the US than in Europe. The pendulum swings, even in the world of computer games.)
This means that there are a number of adventure games which never reach the shores of the US. Even the ones that make it here are often delayed. Traitors Gate was released in the UK months before it was published in the US. Ditto Devil Inside.
Given how difficult it is to find commercial adventure games in the US, avid adventure fans can't ignore what's going on in Europe. If you play a lot of adventure games, you'll soon run through the US releases; after that, you'll have to turn to European release to get your new adventure game fix.
If you're in the US, how do you find out about these Europe-only games? For one, watch the web sites. Many sites, including this one, are still somewhat US-centric, so you'll also need to haunt some of the Europe-based sites. Adventure Gamers is, as you might guess from the name, an adventure-specific site; it's based in Europe, and does an excellent job of covering new releases. GameSpot UK is another good site, and will let you know when English-language versions of adventures are published in the UK.
Once you've found a game you're interested in, how do you get it? One possibility is to order it directly from the company. Many companies have online catalogs, and will ship to the US.
Another is to go to an online software store. Software First, a UK online store, is often a good source of European games. They ship to the US and their prices aren't unreasonable. Also check InteractCD and CDAccess, though their collections of European games is not nearly as good as Software First's.
If you do decide to order games from Europe, there are a few things to keep in mind. One, be sure to check the currency exchange rate. If a game costs £20, how much is that in US dollars? If you don't do the math ahead of time, you can end up paying much more than you expected. Two, shipping costs will add to your total bill. Don't forget to include those in your calculations. Three, sometimes games shipped from Europe arrive slightly more misshapen than they were when they were sent. This isn't always the case, but packages can be harmed in transit. Make sure you're willing to run that risk before ordering.
Those caveats aside, if you're a fan of commercial adventure games, it would be well worth your time to check out some of the European adventures.