Clouds represent a fabulous form of flow visualization that is available to everybody, almost every day. Here are a couple of good cloud galleries:

Cloud Appreciation Society: British site for cloud lovers, including a gallery of clouds that look like things.

Valuca's Strange Clouds: a spectacular collection of clouds, mostly lenticular, with a few others thrown in.

Here are some resources to help you figure out what the clouds have to say about flow physics.

Basic local or national weather:

Weather Underground for Boulder
This site also has surface weather history back to 1986 for the whole US.

AC No. 00-57 Hazardous Mountain Winds & Their Visual Indicators: This is an FAA book for pilots on mountain area clouds and the winds that they reveal. You can download the whole short book.

Current Skew T Plot for Denver.  This shows the current temperature profile in the atmosphere. If the temperature (white line on the right) is steeper than the adiabatic cooling line (solid yellow) then the atmosphere is stable.The 6 am Denver sounding will have a timestamp of 1200z, and the 6 pm sounding will have a timestamp of 0000z, with the next day's date. More info on skew-T plots can be found here. The skew-T for the past 7 days can be found here most easily. Here is a skew-T archive that covers Northern and Central America back to 1999. The plots are a bit bare-bones, but have the basic data you need. The WMO station identifier for Denver is 72469. Be sure to choose skew-T plot as output option.

e-Wall, Penn State's Electronic Map Wall. Forecasters synthesize a local forecast from the predictions of several computer models. This site shows the big model predictions, plus a range of other weather data, such as satellite views and upper air plots.

CDC Map Room. More weather data and model predictions, from the NOAA-CIRES Climate Diagnostics Center, including  polar views.

Satellite images from U Wisconsin, including global montages.

Archives of surface data and satellite images back to Jan 2004, courtesy of U Illinois.

Climate model simulation visualizations from NCAR, including this movie of planet-wide clouds and precipitation for an 'typical' year.

ZeroBlaster: Cool smoke ring toys!

Lava lamp recipes from OozingGoo.com

Cornstarch holes made by vibrating a mixture of cornstarch and water, a dilatant (shear thickening) fluid.

Running on cornstarch: a couple of guys running across the surface of a 1 m deep pool of cornstarch mix. From a Spanish popular science TV show.

High Speed, Schlieren and Photoinstrumentation Photographs by Andrew Davidhazy

There are a number of flow visualization galleries, both on the Web, and in other publications. Here is a sampling:

eFluids: A free resource for fluid dynamics and flow engineering, with a large image gallery, and experiments to try.

The Gallery of Fluid Motion, a juried show of flow images held at the annual American Physical Society Division of Fluid Dynamics meeting. Here is a book of images collected from the Gallery over the past 17 years.

Multi-Media Fluid Mechanics CD-ROM: This CD is full of great images and clips, plus intro-level explanations of basic fluids concepts.

An Album of Fluid Motion, Van Dyke's classic book.

Here are some reference texts, and a few websites on flow visualization techniques: