If you have the latest version of Picasa, namely Picasa 2.5 (available at picasa.google.com), you can find an interesting feature: search your photos by their color. You can type color:green, color:red and any other color from this list: blue, gray, yellow, orange, purple, pink, black and white (I know that black and white are not colors). It would be more useful if the photos were sorted by the predominance of the color, but you can sort them by date.
The filtering is applied as you type, so if you enter color:b, you'll see pictures that contain black spots and pictures that contain blue areas.
This type of search may be useful if you want to find a picture of someone wearing a red dress and the filenames aren't very descriptive.
Another new feature is showing duplicate files. Picasa shows only identical files, so you can delete the copies to save some space. As Picasa doesn't show the duplicate files one next to another, you can enter a keyword from their name in the search bar to show only one set of duplicates.
Picasa is integrated with Google Earth, so you can easily geotag photos (add geographical identification). Just navigate to the location depicted in the photo and press "Geotag". The photo will now have GPS latitude and longitude in the metadata. You can geotag more than one photo at a time. You can also export photos to Google Earth .kmz files, to share them with friends. Geotagging can make finding photos from a specific location an easy task.
Picasa also allows you to upload videos to Google Video. You don't have to visit the site or to use Google Video uploader. You can upload one video at a time.
All these features can be found in the Tools menu.
Related:
Picasa Web Albums
Preloard - online image editor
Use mobile phones for OCR