I've taken a look at Parking in London which is a website that, unsurprisingly, provides you with information about parking in London. However, it's not just London; you can get details on parking places in various other cities as well - I checked Liverpool, Birmingham, Glasgow and even Billericay and Southend had entries for them. The search engine attached to the site is clear, with options to search for city, postcode, type of contract, and rental or sale basis. Each entry gives price per month, address, description, closest station and distance. Price varies wildly, depending on the place that you are interested in - Central London costs are in the region of £400 or more per month, while in the suburbs you may instead be looking at £40 a month. There is an option for 'further details' which includes deposit, price, and an embedded Google map showing the exact location. It's also possible to search by Google maps as well - simply click on the area that you're interested in and zoom in. There's a reasonable FAQ as well.
The creators of the site say that they are "the UK's largest agent for parking spaces and garages" and that wouldn't surprise me, given their coverage. It's a great site to use if you need to find a parking space, have one to let, or want to buy a parking space. It's a really clever idea and if convenience is important to you, it's worth looking at.
[This is a paid review via Reviewme]