Mapping is a key app for any Web 2.0 player, and will become even more important as applications move to mobile devices. Today Microsoft strengthened its hand in the mapping game by buying UK-based Multimap, which is the second-most popular Web-based mapping service in the UK (between Google Maps and Google Earth). The Times of London reports that Microsoft paid $50 million for the tech company, half of which went to founder Sean Phelan.

Other than improving its foothold in the UK (where Windows Live Maps is fifth), Microsoft is also gaining Multimap’s location-based search technology, which can come in handy as it pursues mobile mapping applications. Multimap will become part of Microsoft’s Virtual Earth and Search teams. Its local and geo-mapping technologies will complement Microsoft’s local-advertising efforts as well.

Multimap has been around for more than a decade. It is expected to show revenues of $25 million this year, and be slightly profitable. (It made $1.8 million in pre-tax profit in 2006).