Toronto is home to too many events and festivals to provide a complete list, but some of the larger ones are:

Spring 

Doors Open Toronto (last weekend in May)

Doors Open Toronto allows free viewing of 150 buildings, many of which are ordinarily inaccessible to the public.  One to see is The Carlu: the seventh floor of a former department at College and Yonge Streets.  Designed by French architect Jacques Carlu in 1931, it is considered one of the world's best surviving examples of the Art Moderne Period. 

Luminato (ten days, starting the first Friday in June)  

Luminato is a ten-day festival of art and culture, with a different theme each year.

Pride Week (mid-to-late June)

One of the largest gay pride festivals in the world, Pride Toronto is centred in the Gay Village around Church and Wellesley Streets and features parades down Yonge Street.

Summer 

Toronto Fringe Festival (twelve days, starting the first Wednesday in July)

Now in its 19th year, the Toronto Fringe Festival premiered "The Drowsy Chaperone", a Toronto-created musical comedy that went on to become a smash hit on Broadway and to win five Tony Awards in 2006.  "Chaperone" has meant that scouts for new scripts and future stars are being drawn as never before. 

Summerlicious (seventeen days, starting the first Friday in July)

Summerlicious is a festival of Toronto dining, with many of the city's finest restaurants offering special prix fixe menus for lunch and dinner. 

Caribana (mid-July to early August)

Every summer Toronto comes to life with the excitement of calypso, steel pan music and magnificent masquerade parades and costumes during the Toronto Caribbean Carnival (Caribana).

If you’re planning to visit Caribana, you probably won’t find a hotel room for under $140 in the city.  If you travel a little bit out (for example, to the Days Inn located at the east of the lakeshore) you may be able to get a room for around $99, including breakfast.

Getting around town during carnival doesn't have to be a hassle. The transit system is very safe and cheap. The Toronto Transit Commission has transit maps so you can see how far your hotel is from bus or subway routes. Check the Inside Toronto: Public Transportation page here at TripAdvisor for great tips on how to use the TTC.

In addition to Caribana, Olympic Island offers a festival with Caribbean music, food and local crafts.  The island is a 15-minute ferry ride from downtown Toronto. The island ferry is booked on a first-come, first-serve basis. The stage area on the island doesn't sell out, so you don't have to get advanced tickets. However, they may offer a discounted price if you buy in advance. You should have no problem picking up tickets once you arrive in Toronto.

The Friday before Caribana is a steel-pan competition that takes place at Fort York. It’s called Pan Alive and is a family-friendly event. . It is held in an old fort overlooking the city and there are great views of the city at night.

If you are interseted in playing Mas (participating in the parade as a masquerader), this is a link to a site that has contact info for all of the Mas camps in Toronto: http://www.toronto-lime.com/carnival/...

In the TripAdvisor Toronto forum, you'll find a list of Caribana 2009 official events.  Toronto-Lime.com has an official events list.

For party info, see http://www.toronto-lime.com/events.html and http://www.carib101.com.

Taste of the Danforth (second weekend in August)

Taste of the Danforth is best known as a food festival, but that's only part of this huge street party that takes over Danforth Avenue in Greektown. 

Canadian National Exhibition (eighteen days, mid-August to Labour Day)

An annual exhibition for over 125 years, "The Ex" has midway rides, carnival games, exhibits covering everything from agriculture to technology, and live shows.  As you might expect, the Food Building is home to a massive array of eating options, including a long-time local favourite called Tiny Tom Donuts.

Toronto International Film Festival (ten days, starting the first Thursday after Labour Day)

For film buffs, this is definitely a `Thing To Do Before You Die'. 

Some still debate whether the Toronto International Film Festival has now overtaken Cannes as the most influential film fest in the world (because September is so much closer to Oscar season than Cannes'  dates that Hollywood now finds it convenient to launch some of its biggest flicks at TIFF).   But that's like comparing apples with... champagne, perhaps. 

Cannes is ultra-glamormous, a combination of `official' and `unofficial' film showings, very expensive and overwhelmingly geared to people in the film industry.  In contrast, as a New York Times writer noted a few years ago, TIFF is as `unpretentious' as Toronto is as a city, blending in seamlessly with the city around it.  Toronto will never be as pricey as the Rivera (Roger Ebert once compared the price of a Cannes croissant to that of `a stalwart Canadian bran muffin').  Most importantly for film buffs --- avid locals as well as tourists --- TIFF has never forgotten its origins as a public film festival.  While the industry side of the festival has burgeoned, TIFF still aims to be as user-friendly for members of the public as for industry folk.   Out-of-towners can order even the festival catalogue couriered to them as soon as it is available in early September, and then phone or e-mail for tickets before they get to town.  (For the best prices, order passes or ticket packages by the end of July.)

During ten days in September, TIFF screens over 350 films at a handful of venues in downtown Toronto.  There's something for everyone: star-studded premieres of major films (past examples include Crash, Walk the Line, and Brokeback Mountain), smaller films that have captured the festival's "People's Choice" audience award (Whale Rider, Amelie, Life Is Beautiful), and even smaller films that will never make it to a theatre near you.

The majority of screenings (at least two for each film) are open to the general public.  Die-hard film festival goers spend ten days taking in four or five movies a day, but there are a range of packages to suit those with a more relaxed pace.  For some tips on planning a trip to the film festival, see this thread from the Toronto forum.  Note that hotels tend to book up during the festival, so plan your accomodations early. 

Fall 

Nuit Blanche (last Saturday in September)

Started in 2006, Nuit Blanche is a dusk-until-dawn festival of art installations, live performances and admissions-free museum and gallery-viewing around the downtown core. 

Although planned as a once-only event, it was such a huge, instant hit --- with double the attendees expected --- that City Hall chopped an existing, day-time summer festival to keep this event going.

Cavalcade of Lights (late November through New Year's Eve)

Nathan Phillips Square at City Hall hosts a series of outdoor concerts, fireworks and other events to mark the holiday season.   Cavalcade of  Lights also brings festive light displays to the square and a number of other sites around the city.

Winter 

WinterCity and Winterlicious (late January/early February)

WinterCity brings ice sculptures and live outdoors performances to Nathan Phillips Square at City Hall.  Winterlicious is like Summerlicious without the patios.