The source editor supports command completion. Type the first few letters of a word and hit the escape key. The remaining letters will be entered. Hitting escape again will cycle through all possible completions.

Initially this will have limited usefulness because the completion dictionary is almost empty. To add a word to the dictionary, select the word and choose "Add Word" under the Format menu. Notice that this menu command has a key equivalent. To see and edit the entire dictionary, choose "Open Completion File" under the Format menu. Notice that the completion can be more complicated than just a single word, and the cursor can be placed correctly within this completion. Examine the samples provided in the default file for details.

This feature will be improved in later versions of TeXShop.

Basic Help
Command Completion