Firstly Picasa is not and astronomy programme. Why am I reviewing it then ? Two reasons; you will find it useful in connection with astronomy, and it's a very good bit of software for free.
Having given you the conclusion, what is the software? Picasa is an image managment and manipulation program from internet search giant Google. You may well have a image management program already, particularly if you have a digital camera. Most cameras include a software bundle. So why use a free one from someone else?
The main reason is speed. Piscasa is extremely slick. Once installed it searches your hard drive and catalogues all the images it finds, not just photos you've taken but images you saved from web pages etc. Having done that it generates a thumbnail image for each and displays a list of folders on your drive with thumbnails for each picture. Scrolling up and down the list is very fast.
You can add key words and labels to each picture, which is well worth the initial effort, because as you may expect from Google, another key strength is the searching ability. Again this is extremly fast, with the displayed photos being filtered down as you type a search keyword. For example when I'd typed as far as 'satu' in the search box the programme displayed mainly images of the planet Saturn and its moons. This was before I'd added my own keywords, the pictures were identified from filenames and folder names only, many pictures I'd forgotten I'd downloaded. Now you start to see the use for astromony and space topics.
There's much more to the program though. You can crop and edit pictures directly, including adjusting brightnes levels, colours, sharpness, red eye removal and a range of other filters. All changes are done quickly and effectively.
You can save images for web use, email to friends and even upload to online blogs. Some of the features work best with other google related products and services (such as gmail), but most of these are free as well.
If your hobby extends to astrophotography then the program will certainly help you organise your images and share them with others. You may want to use a more sophisticated package for image editing, but the speed and slickness could well tempt you to use Picasa more than you first thought.
Overall it's an amazing piece of software for free, download it now before someone changes their mind !