For many D&D Gamemasters, this may be an old hat. But I discovered that even the oldest news is still utterly unknown to many, and since finding a decent map (or making one up on the fly because the players are already on their way to the game or decide to make a surprise turn during the session) is such a common concern for all gamemasters, I’ll say it nevertheless:
There is an almost inexhaustable supply of dungeon, castle, town, cavern, ship, fortress, temple, village, sewer and … everything … maps right on the Website of Wizards of the Coast!
It’s a little hard to find since they shifted everything to 4e, but the link to the 3.5 stuff is still there, and maps don’t come with an edition number, right?
I especially recommend this section of the archive with tons of material!
SAGE ADVICE: Since it just MAY be that Wizards will at one point or the other REMOVE the archive in their effort to promote 4e and the 4e online services, I strongly suggest that you download ALL maps you might EVER need!
And if you should ever need MORE maps – or maps that are finetuned and customized to your needs – you can always use the Random Dungeon Generator.