There are many reasons one may want to plan an observing session in advance. Limited time, limited object visibility, making the most of your night out, astrophotography, etc.
Weather & Sky Quality Forecasting
Ventusky (weather models and current conditions)
Astrospheric (mobile app available)
Sky Charts (browser-based)
Planetarium Apps for PC / Mac / Linux
Mobile Planetarium Apps
Astronomy Tools
Online tools to perform a variety of calculations for telescope and astrophotography hardware, and more.