Mashups are the online equivalent of dunking a biscuit in your tea. The combination of data from one site with the data of another site is so much better than each is on its own.
Most of the examples here are still unashamedly US-centric. San Francisco is the centre of the online mashup world, but we're hopeful it won't be long before contributors discover and start catering for the rest of the world.
Microsoft recently launched an alpha version of its Popfly service that should make it easy to build mashups of your own.
The best mashups tend to use maps, such as one of the earliest and best, HousingMaps.com. The site plots Craigslist's homes for sale and apartments for rent on a Google Map and lets you preview a listing by simply clicking on one of the pushpins.
WeatherBonk - Recommended
Get detailed local weather reports, traffic data and weather cameras for cities around the world on a Google Map from WeatherBonk. While it's far from the prettiest mashup, the traffic data could save you hours of frustration, and the webcams let you see weather conditions.
This site lets you submit to Last.fm songs you like to listen to (Last.fm monitors the songs you play on your PC). Pandora FM is a fantastic way to find new music: create a radio station of your favourite songs, then it suggests and plays artists in the same vein.
Pubwalk PubWalk combines bar listings and reviews on to a Google map. Each pushpin has a pop-up window with information, including a rating and a thumbnail picture. Then you can use the service to plot out a bar crawl, print out directions or see them on your phone.
This mashup, cooked up in Google Labs, lets you plan trips by public transport in several US states. Given two addresses, Google Transit will give you directions that combine walking, buses and trains. It even approximates travel times and fares.
Mappr Hackers have done wonders with the open interface of image-hosting site Flickr. But the prettiest of these mashups, Mappr.com, lets you choose a tag and then see all the results plotted on a map of the US using geotags embedded in the photos. While a little slow, searches such as 'beach' or 'Route 66' show the hidden patterns buried in metadata.
Pub Walk can help with the tricky business of finding a watering hole
User review sites
The ultimate recommendations may be the items that come up when you type in keywords about a product type, a service or a destination. There's a lot to be said for popularity. But sheer volume of commentary isn't enough on its own – you need some sort of modifier to make clear whether people are endorsing an item.
This is precisely how sites such as Digg work. A thumbs up or a thumbs down can send an item speeding up the rankings or hurtling to the back of beyond. And you'll notice more and more websites – from Amazon to PCA – including options to rate both comments and products. PCAdvisor.co.uk/reviews encourages user reviews precisely because readers place great store by other readers' opinions.
And while professional critics can be great guides, there's much to be said for the wisdom of your neighbours. Sites such as CitySearch and Yahoo Local have struggled for years to get people to submit reviews of local venues, with mixed success.
If you're heading to the US this summer, we'd thoroughly recommend Yelp.com for its food reviews. It combines a pretty interface; social-networking features that, for example, let you send kudos to other reviewers; and a sense of community that brings food lovers together. Each listing comes with a map and, in the cases of popular restaurants in major urban areas, 50 to 200 reviews with star ratings.
Similar sites exist in the UK. Some, such as Toptable, have user reviews alongside menus, seating plans and discount vouchers, but aren't wholly independent.
Angie's List - Recommended
Angie's List charges US users $6 a month to read and write reviews of home contractors. The number and quality of reviews varies in the 80 cities covered by Angie's List, but larger areas have some very good recommendations. The monthly fee can easily be recouped if you save $100 on a plumber. Unfortunately, nothing of this sort seems to exist in the UK – yet. Word of mouth may be your best bet.
As with other ranking sites, what the UK and Ireland Yahoo site offers in comparison with the US equivalent is poor. Type in 'restaurants' with Chester as the location and, rather than a nice mix of local and national newspaper reviews alongside customer comments, the first three results were for a little American bistro called, er, McDonalds. You can search by proximity or alphabetically, but not by ranking.
CitySearch.com The oldest player in online reviews, CitySearch.com combines paid professional reviews of a wide range of businesses, including restaurants, spas and hotels, with user opinions. Its best feature is the inclusion of handy insider tips, such as which tables to grab for people watching. There's nothing yet listed in the UK, but it's easy to sign up and started adding comments for places you visits – as long as you've got a US zip code, that is.