The first question you need to ask yourself is "what do I need an antenna for?" If you are looking to connect to a nearby access point in the office, then I would go with the Orinoco Range Extender antenna. It comes with an attached pigtail, a stand, and hardware so it can be mounted if required. It is an omnidirectional antenna and can be found on E-bay.

If you are looking for a longer range connection, then you may want to go with a yagi antenna. Word of caution, even though an Orinoco card is low power, it still is Microwave (2.4GHz) radiation. I do not recommend pointing a yagi towards people, or towards a place where people walk by in an office. Most likely there will be no problems, but why take the chance?

The best thing to do before spending any money is to read around on this site (do a search on antenna) and read what different people have done. There are several ways to make your own antenna (pringles can being one) and several other kinds of antennas out there that I haven't mentioned. You may find a better solution than what I have mentioned.

Bill