In my Forgotten Realms campaign we started off playing some D&D modules converted to RM. The second module the players ran though was FRQ3 Doom of Daggerdale.
The villain in this module was Colderan the Razor and when my players ran through this Colderan survived and has started plotting his revenge against the characters. He has been behind several plots now and in when other modules I have used would have introduced another evil magic user I have substituted in Colderan.
In my players perspective this NPC has slowly been revealed to them as the root evil behind countless plots and intrigues.
You may remember back to some plot outlines I have suggested in the past of evil alchemists and how they could stockpile magic in the form of potions or use a range of low level magic to good effect? All these posts were inspired by what Colderan was up to in my Forgotten Realms campaign.
ADDENDUM
I should have mentioned this at the time of writing but it slipped my mind. Egdcltd has just released a supplement on NPCs that fitted in well with the recent NPC related questions.
100 NPCs You Might Meet At The Tavern
Characters will often spend a substantial amount of time in taverns, whether following up leads, meeting contacts, resting or looking for clues and information. Some of the people met may be important for whatever quest the characters are currently undertaking. But what about the others?
Taverns and inns are not frequented just by people who are immediately needed. They will have other customers and staff as well. Rather than simply glossing over these individuals, this supplement provides a detailed list of 100 different people to flesh out a tavern. Each is given a name and described. Some of them may have skills and knowledge that characters find useful and others could be a potential source of adventure hooks.
This supplement currently in the Christmas in July sale on RPGnow for $1.99.