If you're looking for the perfect gift for a runner, here is my list of ideas.

The Stick. No runner should be without the best Massage device. I use mine every day, and it makes a big difference, helping me avoid injuries and easing sore muscles. It's $32 at Amazon.com.
Foam Roller. If your runner already has The Sick, then the Foam Roller is the other great tool for keeping their legs working. It's $25 at Amazon.com for a good quality version that will last.
Books. The recent bestseller 'Born to Run' is well written and entertaining, as well as thought provoking ($11 at Amazon.com).
Halo Hat. This is by far the best running hat I've found, and I wear mine for every run, regardless of the weather ($22 at Amazon.com).
Offer to crew. A different gift is to offer to crew your runner through a race or training run. Crewing an ultra is a big undertaking, so don't make this offer lightly. Another option is to offer to drive so the runner can do a long point to point run.
Foot rubz. While it's not as critical as The Stick or a Foam Roller, massaging your feet with this little ball feels great and is only $6 at Amazon.com.
Sugoi Helium Jacket. This jacket weighs less than 3oz, is windproof, water resistant and is my favorite. (Men's is $54-100, Women's is $65-100.)

Other ideas

These are great gifts, but I'd suggest you ask your runner to find out what they'd like.

ID Bracelet. Hopefully they'll never need an ID bracelet, but it's an important safety measure just in case. There are a number of options available, such as Road ID for $20. However, if they have any medical conditions, I'd recommend going with MedicAlert.
Music. Small MP3 players like the SanDisk Clip Zip ($35 at Amazon.com) or iPod Shuffle ($50 at Amazon.com) are ideal for running. For better sound quality, the most important factor is the headphones, so give a pair of Koss SportaPro ($21 at Amazon.com).
GPS Watch. I recommend the Garmin 310XT for any runner, but there are cheaper options and smaller options, so read Best Running Watch before buying.
Compression Sleeves. These sleeves are all the rage, and for a good reason. They make a nice gift, but they have to be sized correctly, especially the calf circumference, which makes it tricky for them to be a surprise gift. Of the many calf sleeves on the market, I'd recommend the 2XU Sleeves ($40-45 at Amazon.com).

Gifts for Runners