First of all you probably won't get huge results from exercise only at work, but following your criteria, here is what I would recommend:
Use a timer of some sorts. I don't care if it is a manual one on your desk or if it is on your phone. Set the timer for every 30 minutes. When the timer goes off you do as many exercises as you can. Start off with pushups and alternate between narrow, medium and wide grip. After the set up pushups are done (don't worry it won't take more than 60 seconds), move onto the next exercise. Before I got serious and started doing weight training with my power rack, I would walk out the front door to the fire escape and do pullups (while wearing lifting gloves) off of it. If you can do pullups while at work this is awesome. Another great exercise to alternate with is wall squats. They won't replace weighted squats but you can feel the burn if you hold it for a few minutes. There are a lot of exercises you can do for minor muscle groups but since you won't be doing that many sets, I recommend sticking to the big, compound muscle exercises. Just make sure that you don't neglect your core.
If time is an issue at work, alternate between exercises every 30 minutes.
Like I said, your greatest adversary here is your state of mind. If you want to see real results, I recommend starting a fitness program at home. I did p90x at home for about 6 months and got good results. But I got bored with it and am now using a power rack for bench press, squats, deadlift, etc.