Are you talking real world or fantasy? If real world, the link Sapiento provided is probably the best; that's got so much stuff in it by now that it'll probably take you days just to look through it all.

If you had fantasy maps in mind, you can actually create your own base maps fairly easily if you have photoshop or an equivalent. Just use the Clouds filter, then adjust the contrast down to a sharp black and white, and you should have a nice spread of 'continents' and 'islands' which you can then mash up and move around at your leisure.