There are several online guides to Cannes that offer travel tips, reviews and recommendations.
 
Frommer's guide is fairly comprehensive.   There are reviews of hotels and restaurants, and guides to the most popular attractions, nightspots and shopping destinations.
 
The Schmap guide is lots more fun.   You just need to download the free Schmap Player and the free guide to Cannes.   The guide outlines the different neighborhoods and provides pics and reviews on attractions, hotels and restaurants.   Not quite as comprehensive but fun to use.  

The official website for the City of  Cannes has lots to offer as well.  Plenty of event and festival information is included as is a guide to the city including news, local maps and practical information.  It's a good place to check out the local weather too. 

The Backpacker has a one-page kind to doing Cannes cheap.  It's small but helpful.

If you're a filmmaker or film industry professional, or just interested in how the famous Cannes Film Festival works, check out Cannes - A Festival Virgin's Guide.  There's even a great book on the subject.