Haha, it's Windexglow, I see. This is TheCollective15 from the Frictional Games forum. Interesting, being that I was 15 only about...25 hours ago, lol.
In a generic 1990s-modern day office there would be sharpies, markers/dry-erase boards (ranging from four sq.ft. to massive wall-mounted sets), sticky/post-it notes, tape, pens/pencils, highlighters, hole punchers (either bar-shaped or ergonomically-designed, with a protruding handle on the top), plenty of computers (with LCD monitors, likely 15-19 inches diagonal), small speakers, numerous wires, printers (ranging from little personal ones to the massive 500 lb sets for an entire level), fax machines (likely personal-sized), maybe a projector (either mounted on a wall/ceiling or just sitting on a table), some sort of an intercom system, fire alarms/sprinklers, a water jug (one of those with the disposable cups, and the large canister of water on top), clipboards, roller chairs (small ones for the workers, big, comfy one for the boss

), cubicle warfare toys (like little rubber guns or those fighting robots, for geeky workers in their freetime)...this depends on what type of office you are trying to make...if you're thinking of generic Penumbra-creepy-style-office, then I assume you'd tend to keep away from "happy items" (such as the aforementioned cubicle warfare toys), but if you're going to make a Google/Valve-styled office, you should probably look at some pictures under the appropriate image search...
Well, guess I'll hear what this mod will be about sometime later in the Penumbra forums...