This is a brief description of the main tools that I'm currently using for geocaching.
I use GSAK (Geocaching Swiss Army Knife) to centrally manage all of my cache data - it's a bit idiosyncratic, but powerful and useful once you get used to it. Every couple of days or so, I run a macro to download the results of a number of Pocket Queries that I've setup to run regularly on the geocaching.com site. This macro really just uses functionality built into GSAK to update from an email account, but keeps my cache data relatively up to date. From there I run a custom macro to export cache data in a number of formats for use in my iPAQ, and OziExplorer.
OziExplorer is an excellent mapping application that runs on a Windows PC. I export cache data as an WPT file from GSAK, and import it into OziExplorer to explore the terrain, plan routes, etc on my PC.
For car navigation, I use a HP iPAQ rx5965, which is a PDA intended to also serve as a car navigation unit, for which it has a built in GPS receiver and TomTom Navigator 6. The maps are a bit dated, but it generally gets the job done. (TomTom don't supply Australian map updates for Navigator 6 - you need to purchase a later version of Navigator). I also run OziExplorerCE on the iPAQ - this is a cut down version of OziExplorer which runs on Windows CE/Windows Mobile devices, and is useful for getting a wide area perspective while driving, though you need to switch between this and TomTom Navigator.
For the final stages of urban caching, and for bush caches, I use a Garmin Oregon 300, with City Navigator NT Australia loaded as the main map. Garmin now also sell Garmin TOPO Australia maps which provide elevation data and better detail outside urban areas, and directly competes with similar maps produced by OzTopo. I download cache data to the Oregon from GSAK, using a macro which adds a lot of value over just copying a GPX file to the unit.
Garmin MapSource is required to transfer maps to the Oregon, and useful to manage waypoints and tracks. It's also useful for looking at waypoints on a map, though I tend to use OziExplorer more than MapSource.
The Oregon is a neat unit, and pretty much allows paperless caching, with built in access to cache listings, and logging finds in the field. The only real downside is that everything is done through the touchscreen, so it takes a bit of menu work to navigate to different screens - it would be nice to just have a couple of physical buttons to use as shortcuts. GPS Fix is an excellent user site providing information and updates about the unit, in particular check out the Oregon wiki.