A challenge you’ll find here is what does “next” really mean? Since you can tear off tabs, split the UI, even move tabs onto different monitors, I think “next” and “previous” lose a little meaning, unless you know what order they were opened in.
Anyway, some solutions, with older versions (based on when the question was asked) left intact:
SSMS 2008
Ctrl+F6 will switch between two most recent tabs. And honestly, Ctrl+Tab / Ctrl+Shift+Tab work like next/previous except you have to hit Tab twice (you can ignore knowing what the name of the tab in the list is).
SSMS 2012
- Ctrl+F6 will cycle through open tabs in the order they are displayed, and Ctrl+Shift+F6 will cycle in the reverse direction.
- Ctrl+Tab / Ctrl+Shift+Tab will open a temporary window and allow you to cycle through open queries in the order they were last opened.
In more recent decades
- Ctrl+Alt+[Page Up|Page Down] will cycle through windows (as bridge_burner added), but there’s a catch. This only works when the query window is active – and it will stop working if you get to a query window where, previously, you had an item in the grid selected, for example.
- You can make your own keyboard shortcut, as Stuart Smith explains.