I've had some fun over the last year with online riddle games. A couple of years back, I played Qwyzzle after reading about it in Alison Scott's Live Journal. It is an example of the "URL-changing" style of riddles, in which you get a web page which contains a puzzle to solve, and then change part of the URL based on the solution to advance to the next puzzle. As far as I can tell, Qwyzzle is now off-line, though there are many others like it.
Late in 2007, a metafilter posting directed me to a list of room escape games. These are usually built with Flash, and typically have a scenario where you have to escape from a locked room by finding and combining objects and solving brain teasers. Most of them are of poor quality, partly in the game mechanics (pixel-hunting, for example), but more particularly because the puzzles are often rather arbitrary or just daft, and they didn't really hold my interest for long. (With one exception: the submachine series by Mateusz Skutnik).
The same list also included The Roomz. Superficially, this looks like other room escape games, but is really closer to the Qwyzzle genre. It consists of a number of levels, each with a password that gets you to the next one. Some of the early levels are a bit like point-and-click games, but most of them involve breaking codes and solving logic puzzles, sometimes with a bit of lateral thinking thrown in. I don't want to reveal any details, but to give an outline of one "room", you start by seeing some pictures and a string of letters. Working out who and what the pictures refer to gives you a lead in to what kind of code it is and the keyword to break it, and this gets you to the password. This is an early puzzle and is a fairly simple one. By the end (room 45), you are solving multi-layered puzzles, in which you need to crack codes, solve cryptic clues, assemble information from the web (or your own head, if it already contains what you need to know), and make a few inspired guesses. It took me about four or five months to get through the whole of The Roomz, with probably a month or more on the last room alone. I am only slightly embarrassed to admit that I solved the last puzzle while at work, in a particularly tedious meeting.
The Roomz was a good experience, both for the puzzles themselves, and also because it has a supportive and helpful forum for getting you past points where you just can't see what to do next. After I finished it, I cast around for something new to play. Clever Waste Of Time is one that many people speak highly of, but I didn't like that to solve some of the puzzles you end up installing extra software, and also that the forum has a rather hostile feel to it. Eventually, I came across The Labyrinth (and Labyrinth II) at puzzlefiles. In presentation style, it is very different to The Roomz: each puzzle is simply a web page with low-fidelity graphics and a text field to enter the solution. However, the puzzles have the same multi-layered nature, with codes and numerical and word games, and in many cases the need to make an intuitive leap once you've done the initial stage. It also has a forum which, like The Roomz, has people willing to be helpful and constructive. With some of the harder puzzles, I needed to get a gentle hint, then less gentle nudge, and sometimes thundering great shove to figure out what was going on. I finished the two labyrinths after about 7 months, with some longish breaks at various stages.
I found both The Roomz and the two Labyrinths engaging in the same way that a good cryptic crossword in the British style is. They mix conventions (e.g. "blind" often indicates there is some Braille in the puzzle just as broken signals an anagram in a crossword), a set of solving techniques which you might recognize when you see them or might require you to work out something new, and some leap in the dark guesswork. And there is same a-ha feeling when a particularly difficult puzzle resolves. Of the two, The Roomz has richer interaction, though the puzzles are a bit easier and occasionally become formulaic. The Labyrinths have harder and more disciplined puzzles, with a less interesting visual style. I've not yet found a new game to move on to, though some of the puzzles at puzzletome look promising.
(BTW: The Roomz only works with IE. I've had some weirdness in retrying it after recent Flash updates, though not enough to make it unplayable.)