Autofill Series in Excel Spreadsheets

The most useful autofill feature in Microsoft Excel is probably the Series autofill.
If you want to do something simple like a number sequence 1,2,3 etc, enter 1 in the first cell, select the range you want to fill, including the first cell. Then press Alt + H, then F, then I, then S for the Series dialogue, and press Enter.
If you want to do a more complicated pattern, like 5, 10, 15 etc, enter these numbers in the first few cells, select the range of cells, including the first few, and repeat the above. It will realise the pattern and continue it.
You can also do days of the week, months of the year and dates.
To do this, put in the entry that you want to start with, select your range including the first entry, press Alt + H, F, I, S, then Shift + Tab once and Arrow Down to Autofill. Finally, press Enter.