Windows Phone 7: If you’re an early Windows Phone 7 adopter and you’re stuck waiting for the migration of popular apps, App Detective help you set up automatic searches to notify you when your desired mobile apps appear.
It’s as simple as installing the application and adding a query. You can specify how often it will search, restrict the searches to Wi-Fi only, and set other search-related settings such as time of search (setting the time of search to the middle of the night, for example, increases the chance that it will do the searching while your phone is plugged in and charging).
An interesting component of the application is a false positive listing where it shows you really-close but not quite search results that come up for your query and allows you to filter them out.
App Detective is free, Windows Phone 7 only.