Second Skin Labs has a free download of many excellent Windlight presets. The installer can be downloaded at:
After downloading this Windows executable, open it and select the installation folder. For instance, using the Phoenix viewer you would browse and select the folder:
C:\Program Files (x86)\Phoenix Viewer\app_settings\windlight\
Once installed, login to Second Life and select one of the newly installed presets by navigating to the Advanced Sky Editor:
World -> Environment Settings -> Sky Settings
Click the pulldown menu to the right of "Sky Presets" and select one of the entries. I am particularly fond of the Barcelona preset.
Unfortunately, the selected preset does not persist over login sessions so you have to do this every time you login to restore your favorite preset. Let me know what your favorite presets are!