As suspected, Art of the Kill is a very good article on assassins as-they-exist-in-D&D and everything assassin-related. Warning: This is a big article.
Starting with the story-material, we get a brief overview on assassins and their past presentations, a sidebar on secret languages (thieves' cant, druidic, and drow sign), backgrounds for assassins, such as bounty hunter, hired killer, and mystic slayer, and a discussion about alignment that includes a blow-by-blow for how to justify your murderous actions for each alignment.
I'm sure that this will cause all manner of debate and argument to spill forth on the forums, but thats a topic for another post. On the other hand I'd completely forgotten about the "secret" languages, and will make sure to include these in future campaigns where they make sense.
Then we get feats. Loads of feats. General feats for all tiers, Guild feats which kind of work similar to Tribal feats in Player's Handbook 2, weapon-based multiclass feats for the blowgun and garotte wire, multiclass feats that aim to focus your character's fighting style, and finally Technique feats that affect your at-will powers.
I like a lot of these feats, and it really pisses me off when Wizards makes a bunch of content that ends up contesting with other stuff that I was going to take, as it forces me to choose from the absolutely necessary choices (like Weapon Proficiency for the kukri) instead of the other cool stuff (like Eyedark Strike).
Then we move on to paragon paths. There're only two, the blighbeast for the Hot Topic-wearing druid and nightmare weaver for the warlock with Intimidate. Both of these are very, very cool and yet again vie for the next characterI make.
With the blightbeast you can assume the shape of a diseased or almost undead looking animal that doles out hefty amounts of necrotic damage (and can overcome necrotic resistance), knock enemies away with an action point, and also occasionally become a pile of maggots.
The nightmare weaver, on the other hand, emphasizes psychic attacks that can slide creatures about, conjure up illusionary nightmares that impose penalties, and also apply the Rattling keyword to all of your warlock encounter attacks. Very nice, and we aren't done yet.
New weapons include the blowgun and garotte (duh), but also add in some more mundane items like the crowbar (+2 on Athletics to open doors and other locked things), disguise kit, glass cutter, etc. I'm going to make sure to purchase camouflaged clothing for my drow beastmaster ranger (with spider) in Red Jason's Scions of Punjar adventure today. The inclusion of more mundane items that affect skills makes me we want to add in that gnome-enhanced clothing in Eberron that adds to Diplomacy skills...
Finally, the article wraps up with a trio of new alchemical items that can put targets to sleep, blind them, or perform a "ninja vanish".