Dropbox provides cloud-based access to your files — think a flashdrive that you can access anywhere you have internet. I find that it works well as a backup as well as an easy way to share, access, and keep files updated between multiple computers.
Excellent tool for managing and organizing references — particularly PDF’s. Links to Microsoft Word (and supports LaTeX). I think the best feature is Mendeley’s ability to pull metadata out of PDF files and create reference data based on file content. iPad/iPhone and Android support.