Gimp

If you have a pattern that you want to repeat, there is an easier way than copying it time and again: Go to Filters, then Map and choose Tile. Make sure the two scales are connected (i.e. the chain is connected) and switch to percentage unit. Enter multiples of 100 in one of the boxes and press OK.
If you have a texture that you want to turn into a tileable pattern, go to Filters, then Map and choose Make seamless.