If you're into puzzles and ciphers, and want to play the role of a secret agent, then you should give Sentinel a try! Though the game is light on narrative, the story serves to connect one puzzle to the next, as you gather intel and piece together who killed your colleague (and more as the story unfolds). The puzzles are creative and challenging, requiring the reader to conduct research online and also stretch his or her critical-thinking skills. Though this story uses Twine, there are no divergent story paths, rather the system serves to deliver the story and to check puzzle answers. I confess that I needed more hints than I care to admit. There are more puzzles to come, so the current conclusion is a little anticlimactic. I need more puzzles to solve and for the story to come to a resolved conclusion!
UPDATE: Many more puzzles have been added, and though I think some are very difficult, I really enjoyed playing them. Just remember that this is not 'traditional' IF. It is a puzzle / cipher adventure that is delivered via Twine.