The Venice Card is the smartest purchase you will make before visiting Italy. The Venice Card combines transportation, toilets, and museum entrance fees into one handy card. Not only is it a good deal, it is also much more convenient than fumbling for Euros.
There are two different levels of the Venice Card. Blue cards offers access to public toilets and museum visits. Orange cards add unlimited transportation on any vaporetto line. Either one can be enhanced with Alilaguna transportation to/from the airport.
The Venice Card can be bought for one day, three days, or seven days. It comes with a little booklet and a map. Actually, we didn't get those, but that was because I screwed up our cards (see Tips, below).
We did not use the public toilets, but we did use our Venice Cards to avoid the lines at the Doge's palace. The other museums included are: The Museums of Eighteenth-Century Venice, and the Island Museums on Murano and Burano.
The Venice Card also comes with some extras. For one, anyone under 29 gets the Venice Card Junior, which includes all of the Rolling Venice benefits.
Senior members (anyone over 30, sorry!) will get free access to the casinos on the Lido and in Mestre.
The card also includes discounts at shops, hotels, and restaurants. See this document for more details. It actually included a discount for my Hotel, the Bernardi-Semenzato, but I was not aware of this at booking time. Any place with a Venice Card logo will give you a discount. Also, you get a discount on the Chorus pass, which is like a Venice Card for churches. Finally, the Venice Card entitles you to some discounts in the old Jewish ghetto, at the museum and its shops.
You may choose to buy online or merely reserve online. Either way, your cards will be waiting for you either at the airport or at the train station, depending on your mode of arrival. Buying online represents a slight discount.
Check the Venice Card site for updated prices. When we visited, the three-day Orange cards cost 47 for a senior and 35 for a junior, not including the Aliguna transportation. It doesn't actually cost less to purchase the Alilaguna transportation separately, so that's what we did.
Be very careful when purchasing the card online. If you select the wrong dates, as I did, your card will not be valid. Fortunately, the website had been broken and they had lost my reservation, so after a long conversation in mixed Italian and English, I was able to get a new card. It was, however, quite a hassle and I had to go to Tourist Services because the Venice Card people wouldn't help me at first. So, don't make my mistake. Get the dates right the first time. I had reloaded the page, which reset the dates, and hadn't noticed it when filling out the form again.