For those new to IF, I like to recommend Photopia [1] as a second game. It's less traditional in format, but it's a great example of the kind of storytelling that the medium can support. Shrapnel [2] is another game somewhat along these lines, but it takes things in a different direction.

Once you know your way around the command parser, I suggest Anchorhead [3]--especially if you're a Lovecraft fan--for the richness of the atmosphere it creates. Slouching Towards Bedlam [4] is another good entry in the mystery/horror genre, featuring steampunk themes.

More games, plus ratings, collections, and reviews, can be found on IFDB [5].

EDIT: The Act of Misdirection [6] deserves a mention, too. Another horror(-ish?) game, and the one that got me hooked on IF after Zork and Adventure.

[1] http://adamcadre.ac/if/photopia.html

[2] http://iplayif.com/?story=http://mirror.ifarchive.org/if-arc...

[3] http://pr-if.org/play/anchorhead/

[4] http://ifdb.tads.org/viewgame?id=032krqe6bjn5au78

[5] http://ifdb.tads.org/

[6] http://iplayif.com/?story=http%3A%2F%2Fwww.ifarchive.org%2Fi...

A wholehearted +1 for both Phototopia and Anchorhead - these are both excellent examples of IF. It should be noted that Photopia is much easier and shorter, being more of a slightly interactive short story (but what a story!) Anchorhead is much tougher to play through, though very rewarding.