While Venice may not excel in the food stakes, it more than makes up for it through its wonderful bacari. These bars attract young and old alike from all walks of life and are reminiscent of the old British pub before it became a coffee shop and creche. The bacari sell wine (often with extensive selections of some good quality offerings) and cicchetti, although beer and coffee are also available. Wine is usually served in large elegant glasses with a reasonable 150-175 ml offering. 
Some of the bars offer a wider, more sophisticated range of wines and fall into the category of enoteca or "wine repository" offering high end tastings of the local wines. In some cases it is possible to buy bottles of wine to take away. Enoteca can be recognised either because they have the word in the name of their establishment or will have a sign in the window or door advertising the fact. 
Venetian drinking centres around the "ombra" a word that literally means shade. In olden times a trader might take some refuge from the sun in a local bar or osteria to take a little ombra. The name is now associated with a small shot of wine and so to order a drink in a bacari, you simply ask for an "ombra di bianco" or "ombra di rosso". Depending on the establishment that will get you either a small tumbler of house wine for 2 euro or so or you'll be asked which wine you want, get a nice glass and pay around �3-3.50. In an enoteca it is possible to pay up to �10-15 a glass for some serious wine (but it's worth it). 
A popular pastime is the "giro di ombra" or wheel of shades which is, in effect, a pub crawl. Thankfully, a giro di ombra is not a binge drinking nightmare but a civilised affair involving a trawl around some excellent bars taking a couple of drinks in each together with a variety of cicchetti. 
Here are some of my favourite, tried and tested, bacari: 
Antica Osteria Ruga Rialto (Ruga Rialto, San Polo 692) 
If you like a drink this fabulous bar is a "must do". A cosy, rather eccentric, bar that attracts locals of all ages. It serves a good selection of wines including the classic house ombra (�1), beer and spirits. An interesting selection of cicchetti is available including local sausage, pickled raw fish along with the usual selection of meatballs and tuna rolls. 
Whilst you may believe that the bar is run by the friendly guys behind the bar, the real master of the establishment was "Kiko", a large black cat that sa or slept on the shelf behind the bar. He looked the establishment over and occasionall. I mentioned that he seemed to have a good life to the one of the guys - "Like a king" was the response. Sadly, he is no longer with us.
Open from 12-2.30pm for lunch including hot meals such as frito misto and from 6pm - 1am when cicchetti is still available.  Closed on Thursdays.

Un Mondo di Vino (Salizada San Canciano, Cannaregio 5984) 

This busy little bar is open all day from lunchtime onwards and serves some excellent wine. I had a superb sauvignon blanc for �3.00 but in addition to the good wines the cicchetti is extensive and interesting. A good point about the food is that a lot of it is fresh, not fried. Featured items on my last visit were sardine fillets in saor, cheese wrapped in zucchini, chicken brochetta along with the usual rice and tuna balls. 
It is friendly and busy in the evenings and popular with people living nearby. Occasionally the customers spill out onto the pavement creating a nice bustly feel to the whole place. Visitors are made to feel as welcome as the local people. 
Closed on Mondays. 
Al Merca (Campo Cesare Battisti, San Polo 213) 
This is a tiny place where you eat and drink standing up in the open air. Al Merca is a popular bar situated in a square immediate adjacent to the Rialto vegetable market. There is an extensive selection of wines available ranging from �2-10 a glass and, as ever, a spritz can be mixed. The food is very good and includes the usual polpette (meat and tuna balls), seafood antipasti, fried fish and small rolls with a selection of fillings. The rolls are made on the premises which is a miracle given the size of the place. 
I would recommend taking a restorative drink here after a stroll around the fish and vegetable markets. Open until the early evening (8pm?) 

Cantine del Vino già Schiavi, Dorsoduro 992 - fondamenta Nani

A most amazing bacaro and wine shop situated in the Dorsoduro neighbourhood. It has a massive selection of interesting wines that can be bought by the bottle and many can be sampled by the glass. The men look after the wine while Mama oversees a big selection of cicceti. Rumoured to be visited by "A Listers" staying in the posh hotels on Giudecca. Warning: there are no chairs, just a low wall outside. Much favoured by students from the nearby University and very close to one of the few remaining squeri (boatyards) in Venice

Cantina do Mori (Calle do Mori, San Polo 429) 
This is a great little bar close to the Rialto Market. Pop in (as the market traders do) for a quick glass of wine from its extensive range. It also features some good cicchetti to go with your drink. Work on �4 for the wine and �2 for a piece of cicchetti. 
Pots and pans hang from the ceiling, service is brisk rather than friendly (some say cold) although asking for a recommendation tends to break the ice. There are (at the last time of visiting) only four chairs (an increase of one since Feb 2009!) in the main bar and a small shelf on which to rest your lunch. There are sometimes tables to be had outside. Not to be missed.