One of the best and most popular places to find last minute tickets is the website lastminute.com. Not only does it have offers for flights and hotels, but also for the Eurostar, car-hire, restaurants, spas and general entertainment. It allows you to filter your results by date, destination and various other options, and sports last minute deals for rock bottom prices. It also has late deals with even better prices if you're flexible enough to book then and there. The site is well laid out and easy to navigate
Another great place to go for last minute tickets is travelrepublic.co.uk, which offers amazing deals on flights and hotels. They list over 100,000 hotels and 100 airlines, and guarantee that they'll have the lowest prices. For example they currently list the Trh Jardin Del Mar Apartments in Majorca for as little as £8 a night. The site doesn't have quite as many options as lastminute.com, as it lacks any deals on entertainment, restaurants or spas, and doesn't allow you to filter your results in as many ways. But if you're looking for a holiday abroad then its offers are just as good if not better.
There are of course a number of other places you can go for last minute deals, and a quick internet search will turn many of them up. However the two sites above both have competitive prices and a fairly comprehensive list of offers. They are also both very popular and well liked by customers.