Google Insights recently rolled out a new feature: 12 month search forecasts. The forecast comes from a relatively simple decomposition of the search volume into trend, seasonal and residual components. The model's out-of-sample performance is tested on the most recent 12 month period; if that prediction proves accurate, then the model is accepted. Here's what it looks like when searching for "Google" (blue), "summer" (yellow), and "weather" (red):
The "Google" volume shows a clear macro trend with little seasonal impact; "summer" is of course the opposite. "Weather" proved too unpredictable and no forecast was generated.
A complete description of the methodology is available in this paper by Google (pdf link).
Click here for a live view of these trends.