I love letterboxing, and often describe it as treasure hunting with no treasure. Well, geocaching has the treasure. Well, at least it does if you consider small toys and trinkets treasure. Whatever. It is just as good.
Geocaching uses GPS coordinates instead of written clues, and is much more widespread. For example, there are 10-20 letterboxes in my metro area. There are more than 200 geocaches. I will never find them all. Which is a good thing.
The only real requirement is a GPS unit you can plug coordinates into and a computer with which to obtain those ingredients. The best site is the official geocaching site, www.geocaching.com. It is a free membership and there are thousands and thousands of caches everywhere.
I have added my GPS to my letterboxing kit and plan to look for them wherever I go. I recommend you do the same. :)