Quite a bit depends on whether you are shopping for a man or a woman. There will be different preferences for each. A woman, for example, will likely enjoy a spa gift certificate, while a man probably won't be all that enthusiastic about the idea.

The other big factor is your budget. Are you shopping for something just from you, or from other coworkers as well? Obviously a group gift will generally have a better budget and you will want to choose something that everyone will be happy about giving.

Gift certificates are something of a perennial favorite. Think about dinner for two at a good restaurant. You never want to do it for one, since most of your coworkers will have somebody they can invite to come along.

Houseplants are a classic idea. If you want to go a bit different, take a look at bonsai rather than the typical houseplant. Bonsai trees can come in beautiful pots and have a great style to them.

Books can be a good choice. You can go with something on professional development for their career, but that may not get a very enthusiastic response. A little career humor, such as Dave Barry's "Claw Your Way to the Top: How to Become the Head of a Major Corporation in Roughly a Week" may be much more fun for them. Other books may also be worth considering.

Some offices have a little fun. Fun gadgets for cubicle warfare or for the geeky side of people may also be a good idea. Just keep it to a level appropriate to your office.

Gift baskets are another great idea. You can buy ready made ones or customize one without an excess of effort. Fine chocolates for the chocolate lover, gourmet coffee for the office coffee fiend, and so forth. If you know they love something you can probably make a gift basket themed around it.

Engraved gifts are also quite popular. It can be a lovely way to commemorate an achievement.

Perhaps the most important thing is to not get too personal in the gift selection. This is a coworker, not a close personal friend. You probably shouldn't humiliate them or embarrass them. You don't want to give something and find out that they have a dozen at home already. Keeping things on a relatively professional level will help to keep you from that kind of mistake.