Necromancer games has a number of good dungeon modules, almost all of which can be used in your average campaign. Some favourites:
-The Tomb of Abysthor: fairly low level multilevel crawl, with different factions you can fight or ally with, including two different varieties of evil priests, an undead wizard and a drow woman. Pretty cheap for its page count. It is out of stock for WW, but you may still find it at various distributors.
-Crucible of Freya is a wilderness adventure, but I found it excellent to start (and, in many cases, end

) a campaign. Moreover, it is extremely easy to drop just about anywhere where a small village and its environment can fit.
-Hall of the Rainbow Mage has two dungeons, each with a different flavour (one an abandoned mages abde, another a multilevel evil temple)
-The Vault of Larin Karr: describes a valley with multiple villages and a huge number of mini-dungeons, many of which are connected through the Underearth. Very inspired, has several hours of adventuring potential.
-The Lost City of Barakus: upcoming (should be out this month!), huge book with a ruined city and many sub-adventures.