I am summarizing the most useful Eclipse shortcut keys which are really helpful in day to day work
Ctrl + Space : Content Assist
syso + Ctrl + space = writes out System.out.Println for you (I love this one)
Ctrl + Shift + T : Open resource box to look for a Java file
Ctrl + Shift + R: Open resource box to look for a non Java file
Ctrl + F8 : Switch between perspective
Ctrl + F11 : Run the last launched
F11 : Run the last launched in debug mode
Ctrl + Page Up / Ctrl + Page Down : Move between open classes
Alt +Shift +J : Add block comments to a clas sor method
Ctrl + Shift + / : Comment the selected code
Ctrl + Shift + \ : Un comment the selected code
Ctrl + / : Add or remove line comments
F3 : open declaration
Ctrl + Shift + L : See all the shortcuts in eclipse , if you do not remember anyone of above 