GIS events come in a few flavors. There are broad based shows that cover many aspects of GIS use including the annual GITA and URISA Conferences held in the US. Comparable conferences in the UK include GeoSolutions and in Europe, INTERGEO. These tend to cover many topics and have the support of a variety of vendors. Other shows explore GIS use in a particular discipline such as Oil and Gas or Forestry. These tend to be smaller, but more focused. Some conferences focus on one technology, such as mobile computing or location-based services, or imaging.
Even more specialized are the vendor specific conferences that cover all aspects and uses of a single vendor's product line. These tend to be more training focused and more, rather than less, technical. Some, like ESRI's are huge. Others, like Autodesk's include not just GIS users, but all of the company's software users. Intergraph has a specific show just for its GIS users. In 2004 MapInfo is having a few short conferences in place its traditional weeklong event.
The trend I see involves smaller, shorter more focused working conferences where the aim is as much to network as to get "work done." Conferences specifically on GIS and CAMA, or addressing or policy conferences such as NSGIC (midyear) fall into that category.
Also, consider online events. Some "marketing" events can be very educational (and free). Some organizations, GITA comes to mind, host fee-based sessions on a variety of topics throughout the year.
Conference registration, airfares and hotels are expensive, so its worth digging around to find events that match your needs. How? Look at the program - are there more than one or two papers relevant to your needs? If not, keep looking. My advice, after attending ten years of CAD and GIS conferences, is this: when in doubt, go to the smaller events.
There is alas no single overarching event directory to consult covering GIS events worldwide. Together, these few cover a good percentage: Directions Magazine, GeoCommunity, GISDevelopment.net, GeoWorld.