It's easy to see why because geotagging photos is fun!
Just look at Terry White's post, Nikon D300 + N2 di-GPS = FUN!
If you shoot with a Nikon DSLR, there are several options for geotagging photos.
The N2 di-GPS that Terry reviews is one. It's $180 US including shipping, and as you can tell from Terry's post, he likes the N2 di-GPS.
Dawn Technology Limited, the makers of the N2-di-GPS also have a pro version called the di-GPS Pro L with a built in data logger.
Nikon's own GPS Unit, the Nikon GP-1 is another option. At just over $200 US, it's about the same price as the N2 di-GPS, and you can't go wrong buying Nikon.
The Promote Systems GPS is another option for Nikon shooters that works with the Nikon D300, D700, D3, D2X, D2Xs, D2Hs, D200 and Fuji S5 Pro DSLR Cameras. At $149, it's a fair bit cheaper than the Nikon GP-1 or the N2 di-GPS.
Gerard Prins over at Nikon Reviews in a Nutshell gives the Promote Systems GPS-N-1 a Highly Recommended in his review "Geotagging" with your Nikon. Review of the Promote GPS N-1.
The GiSTEQ PhotoTrackr is another option for Nikon DSLR cameras that may be your best value at the moment. Amazon has it on for $79.99 as of now while I'm writing this post.
Ben Spark has a review of the GiSTEQ PhotoTrackr on his blog, www.benspark.com, which he sums up by saying "The GiSTEQ™ PhotoTrackr™ ROCKS! And the GPS is very accurate."
If you want to get into geotagging photos with your Nikon DSLR, these are some great options to consider. Obviously your budget is going to weigh in heavily on what you choose, but the GiSTEQ PhotoTrackr looks like best value for geotagging photos at the moment.
** Update - New Geotagging article - Geotagging Photos and Nikon DSLR GPS Hardware and Software Comparison by Dierk Haasis**