The ultimate online resource for information on ZX Spectrum computers is undoubtedly World of Spectrum. There you can find an enormous archive of software and documents. Thanks to Martijn van der Heide for organising such an impressive collection.
The HTML version of the original ZX Spectrum manual may be of particular use if you are new to the Spectrum or your memory needs refreshing.
The World of Spectrum forums are a fantastic place to read about what others are using their ZX Spectrum computers for and to find help with any problems you encounter.