Since I've been thinking about this a lot lately, it seems useful to at least set these thoughts to paper. There seem to be a number of nondiscreet, overlapping roles that various NPC information sources can play for a party. In some games (like Shadowrun) these are explicitly spelled out and are part of the setup when starting the game. However, unlike those games, D&D allows you to take everything in hand yourself—your DM needn't even necessarily include figures such as these, but I find it allows everything to go a bit more smoothly plus it gives that sense of realism rather than allowing hoards of information to fall willy-nilly into the PCs laps just because they are players.
Rattling off the top of my head, here are some jobs that NPCs generally do for PCs:
Employer. The simplest and most straightforward of the relationships is employer-employee. This is easily complicated, however; many important people won't employ adventurers directly, but rather delegate the task to an underling. So now you have a pipeline to your employer, via a spymaster, seneschal, or simply a trusted advisor. Employers can betray you, or be employing you to a task they don't ever intend to see done, or have completely ulterior motives, realizing that the chain of reactions set off by doing a job will be one that's beneficial to them.
Contact. These types of NPCs are intermediaries for jobs, not in the way an employer's agent is, but rather as someone who knows what needs doing and can hook the player up with many employers. This is generally a person you want to trust. PCs can't be expected to sit around and build contacts with all the important people of a city all day while they're off busy adventuring—though they are certainly welcome to, and their contacts will likely wane in importance to their own social networks as they develop roots in a locale.
Fixer. This covers a large variety of needs, but generally is a person with a single specialty that the PCs hire to make use of that specialty. For example, a man who has influence with the underworld may be able to drum up extra muscle for the PCs. It's highly unlikely that the PCs are going to want to spend time going from one group of thugs to the next to see if they want work that evening. This is left to a professional organizer, the fixer. These NPCs usually perform their service for other NPCs, and only work for the PCs temporarily.
Broker. Information is important, and getting information that others don't have is the MOST important thing there is when you're adventuring in dangerous political waters. Spymasters generally work for a noble house or organization (the crown, perhaps) but information brokers are free agents who make it their job to know things. Many brokers are only informally in this business: a wealthy merchant who just happens to know a lot of people might serve the PCs as a good broker.
Now note that there is a huge amount of crossover between these categories and that NPCs may start as one and become another—for example, it's an easy transition to work for a merchant, impress him with the results of the job, and then return to him again and again for information. This is an effective network-building strategy for adventurers: turn employers into other kinds of long-term contacts, in this case employer->info broker.
Most people who can serve as employment contacts can double as information brokers as well, however limited their scope. Remember, this is probably a fantasy/medieval setting we're talking about, so most people are highly specialized. It's a rare man who makes his living doing all kinds of employment contracts (the equivalent would be a modern day acting agent; no one has the free time, nor can the economy support, adventuring agents who go around and look for work for their client-parties).
It's also important to realize that all of these come in both licit and illicit varieties. You may befriend the king's spymaster (a great info broker) or pay a local crime lord: both will wind up giving you information, but it will be of different provenance, deal with different subjects, and be of varying quality.*
These categories aren't really tools to use as labels when you create NPCs, but rather simply types of resources to keep in mind. Any NPC could be potentially all four of those things to your PCs... It just depends on how they interact with them and if the NPC is providing the right kind of service. It doesn't help the PC who wants to hire a cadre of assassins if they're speaking to the knight-captain of the realm.
*The misinformation issue is something I love, though my players have become extremely paranoid. Misinformation is rampant in settings where the only means of newsgathering is word of mouth and you can essentially determine that the farther away the source of the news was, the more distorted it will be when it reaches you.