A Stammtisch is the table at which regular customers meet. In Germany, it's common for groups of friends or people with common interest to get together on a regular basis. English terms that might encompass similar activities would be special interest group and bridge club. Stammtisch often gets directly translated as "regulars' table".
In Germany there are many, many Stammtische. Some are groups of friends that have been meeting since they were kids and some are people with a common interest such as a Motorrad Stammtisch ( motorcyle enthusiasts meeting ).
Stammtische generally meet at bars and restaurants, so it's a social eating and drinking activity.
There are several Stammtische in the Phoenix area.
The Tempe Stammtisch is probably the oldest Stammtisch in Arizona as it has been meeting every Friday since the early 70s. It's a german-language Stammtisch that meets every Friday at 20:00 at Boulders on Broadway.
The Free Software Stammtisch has been meeting monthly for more than a decade. It's a Stammtisch for people interested in Free Software like GNU/Linux and *BSD. It meets at 19:00 on the 3rd Tuesday of every month at Iguana Macks in Chandler. Sometimes refered to as the GNU/Linux Stammtisch. There is also a Free Software Stammtisch in Prescott. Generally everybody speaks in English, but other languages are not discouraged.
The Geschäftsstammtisch meets about once a month to do business networking in German. It generally meets at a different location every month. One is expected to speak German at the Geschäftsstammtisch.