If you're an Android phone or tablet user, you'll be interested to take a look at LEGO Collector, which is a "unique application to browse and mange your entire LEGO collection including sets and all minifigure series."
It interfaces with Brickset's web services so you can download your collection to your device and keep it in sync.
I've installed it on my Galaxy Nexus and it works well, and pretty quickly too. My only criticism is fairly minor: it allows you to download instructions to your device, to the SD card, but once it's done so, unless I am missing something, they are hard to find and view. You probably need a third-party file manager to get to the contents of the SD card, which I don't currently have.
Other than that, so far, it's been faultless. It costs 84p (probably $1 and €1).
The mobile app directory, as always, contains details of all apps that work with Brickset.
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