http://www.gsak.net/GSAK is the program I rely on most to help me with geocaching, so I've got a lot to say about it. In fact, I will ramble here, and this will probably be about half the stuff I do with GSAK.

There is a bit of a steep learning curve when first using GSAK, but all users should carefully read through the GSAK 101 document to figure out how to use the program.
If you are computer illiterate, this program probably is not for you.GSAK is basically a database where you can load cache files (GPX files, pocket query results, etc.). Once caches are in the database, you can do dozens of things. You can add "corrected coordinates" to solved puzzle caches or caches with bad coords. You can set up filters to view a subset of the caches (events only, unfound caches, unsolved puzzles, you name it . . . there are thousands of possibilities). You can temporarily change your "center point" to view the caches nearest a different location (excellent for traveling).
You can upload the current filter of caches to your GPS. When you upload to your GPS, you can have GSAK rename the waypoint and description: I have the cache titles as the waypoint names so the map shows cache titles. For the description I have the GC code, the cache size and type, the terrain and difficulty ratings, and the last 4 logs (found or DNF). With those settings, I have lots of useful information just by looking at just my eTrex!
This is one of those tools that gets more and more useful because you keep learning about new features and abilities. You can also download macros that give you additional functionality.
Here are two macros I use regularly:
- FindStatGen3: This macro gives you very powerful statistics on your finds and your hides. The first time you run this, you'll be blown away by how much information this macro shows you! You can look at my profile to see the output from this macro. (The stuff in the table/grid is from FSG3, the stuff at the bottom is not.) Some of the major items you can view:
- Finds by month
- Milestones
- Distance from home
- Cache Type
- Diff/Terr
- Owner (of found caches) stats
- US state, Canadian province, European, Australian, and other maps
- Days of the week
- Container type
- FTFs
- Bearing (direction from home)
- Place (Hide) stats
- Challenge: One of several macros that give you your status on the Delorme challenge. Just pick the state you're working on and the macro will give you one GC code for each completed grid. You can choose to have the macro open up Google Earth (if you have it installed), zoom in on New York, draw the Delorme grids over New York, and highlight your completed grids.
The program is free, but nag windows come up after 21 days. Runs on Windows platforms.