THREE DAYS TO KILL by John Tynes from Atlas Games; the first d20 third party published adventure. The PCs are hired to assassinate a specific target without causing too much carnage. Lots of good role-play opportunities, a colorful cast of NPCs to interact with, all in a great "party-town" (holding a festival of sinful delights!). Very nice module for any non-good PCs (esp. those with stealth skills), as it involves murder for hire and unscrupulous NPCs with sinister motives out to use them.
Lots of good ideas in this thread, too. Evil groups often battle other evil groups even more than good ones, as the evil groups clash over the same or similar goals.
Demons and Devils hate each other, while Daemons (Yugoloths) change allegiances at the drop of a coin, playing both sides up the middle. Evil gods have many enemies... In FR, Bane hates Cyric, and both seek to destroy each other utterly to become the "Tharizdun" (One True Power of Evil) in the Realms. Loviatar and Talona have a deadly rivalry, esp. since Talona left the Dark Gods pantheon headed by Bane and defected to Shar's camp.
Having your PCs be agents of the Zhentarim also could put them into the middle of a Banite/Cyricist faction war around Darkhold for control of that great fortress. Manshoon might need pawns to retrieve this artifact or that for his sinister researches or to work towards revenge at a later date on Fzoul for usurping his control of the Zhentarim and Zhentil Keep. Red Wizards could hire the PCs to capture some horrid monster for their sinister experiments...
If you can, get the PCs to join some larger group for wealth, prestige and protection... advancement opportunities often come only at the expense of other members of the larger group, and if the PCs aren't at the top of the pecking order (which should be the case), they get sent out on missions by their superiors.
I've found that having a rival group of NPCs that mirror the PC's own is an effective tactic, as is a group of opposed goody two shoes NPCs out to get both evil groups. You can get a lot of mileage out of this situation, with members defecting from one group to join another, forbidden Romeo & Juliet type romances, etc., and there is great satisfaction in killing a rival or one who has hounded your footsteps in the name of justice over some petty crime (such as murdering his family) that you can barely remember taking part in, lol.