Most game links go to the relevant entry of Baf's Guide to the Interactive Fiction Archive.
Kaged by Ian Finley. Orwellian dystopia, anyone?
Nevermore by Nate Cull. A nice interpretation of Poe; heavily dark and metaphysical, and with lots of drug abuse. Some of the alchemy is a little excessive, though.
Voices by Aris Katsaris. A dialogue-driven interpretation of the Joan of Arc story; perhaps a little constrictive, but adds some interesting angles and does a good job of raising questions. The paragon example of IF dealing with religion that works well.
1893: A World's Fair Mystery by Peter Nepstad. Huge, professional and sprawling, this is basically a simple puzzlefest writ exceptionally large, and the setting is really the star. Character, prose and (to an extent) plot take a back seat.
Edifice by Lucian Smith. A clever evolutionary fable, with one much-vaunted puzzle. I liked the first section the most, and thought the last section uninspiring, but overall it's a worthy piece.
Worlds Apart by Suzanne Britton. Big, very polished and well-written, if a little bit too New-Agey in flavour for my tastes.
First Things First by J. Robinson Wheeler. A sizeable and highly engaging (if somewhat oldskoolish) time-travel puzzlefest.
Rameses by Stephen Bond. Linear and with limited interactivity. Deeply angsty.
The Tale of the Kissing Bandit by J. Robinson Wheeler. Linear and somewhat sparsely implemented, but very funny indeed.
Textfire Golf by Adam Cadre. Hilarious. Essentially a golf simulation, with some evil twists; I have no interest whatsoever in golf, but became addicted to this.
Shade by Andrew Plotkin. Surreal and troubling, although it takes persistence.
Babel by Ian Finley. Pretty much the archetypical amnesia-in-science-lab game. Dark, and with perhaps over-the-top religious overtones; the man-should-not-meddle theme has been done so much it's hard to come up with much new, but it does well for what it is.
Rematch by Andrew Pontious. A brilliant and intricate exploration of the one-turn game. Difficult, but rewarding.
Kissing the Buddha's Feet by Leon Lin. This starts out on a fairly basic dumb-student-house premise, but develops a sense of chaos very pleasantly. Simple puzzles, and nothing very deep, but amusing and responsive.
Necrotic Drift by Robb Sherwin. A lot more polished than Fallacy of Dawn, but ultimately not as great a game. Geekery abounds.
Christminster by Gareth Rees. One of the paradigm Middle Period games, it's often unfair and awkward but is a great game underneath. Extra points for Oxbridge setting.

Other recommended-IF sites:
Baf's Guide to the Interactive Fiction Archive is an essential resource.
Chrysoula Tzavelas' IF Ratings site is a public voting system that offers a list of highest-rated games. (The more people who use it, the more powerful it becomes, so if you're feeling public-spirited get an account and rate games as you play 'em). The highest-rated games are generally worth playing, if only so you can have an opinion of 'em.
IFwiki is, well, a wiki for IF. Emily Short's site has a fairly comprehensive listing, intended to list games which employ interesting techniques rather than a straight Best Of.
Suzanne Britton's recommendations are definitely worth a look, although the page hasn't been updated for a while and hence won't contain new releases.

You Are A Chef! by Dan Shiovitz. A massive source of injokes; leave sanity at door.
The Lion In Winter by Iain Merrick. Very funny, contains several reasonably good puzzles, and has a coherence that most speedIF ends up lacking.
The Relentless Adventures of Captain Speedo, Episode 16: Let them "heat" cake! by Gilles Duchesne. A good example of speedIF wackiness; the Captain Speedos are in general pretty good, and this one just happens to be my favourite.
A Day For Fresh Sushi by Emily Short. Extremely competent and detailed for the constraints; a particuarly good example of effective NPC use.
Freedom by Peter Berman. Pleasantly tongue-in-cheek, decently realised.
Reality Show by Peter Berman. As far as I'm concerned, this has two major advantages: philosophy jokes and a cameo by yours truly. Flattery will get you everywhere, sir. My major issue: this is an existentialist game, it features coffee, and you can't drink it.
Digging for Onions by Christopher Huang. Manages some pretty darn tricky things within the constraints, and comes off feeling poignant. Nice ideas flying around here.
Useful speedIF sites:

Gunther Schmidl maintains an archive of every speedIF ever.
SpeedIFs tend be difficult to win if you don't get the idea immediately; David Welbourn has solutions to every speedIF (among other things).