You should be able to run most existing 3.0 low level modules with 3.5 without skipping a beat -- after all, if an NPC's skill ranks are off slightly, or he's a 3.0 instead of 3.5 ranger, will the party really notice?
And since so many of them dispense with statblocks to refer directly to the Monster Manual, and monster changes are alredy taken care of!
Just make sure to double check that the monster in question didn't increase in power. I can't think of any low-level critters that got too big a boost, but something like sending a 3.5e mummy or osyluth instead of a 3.0 mummy or osyluth* might be kind of unintentionally fatal for the PCs.
*I'm thinking of one 3.0e 5th level adventure in particular.
Ooh I played in that adventure. Not with anyone on these boards but in the adventure. Glad you brought it up the 3.5 version of the osyluth would have slaughtered us. I mean only 1 person in the party would have been able to deal enough damage to bypass the damage reduction. Even then 1 or 2 damage hits would be slow with its 90 something hitpoints.
I'll say what module it was but I don't want to spoil anything.
The Crucible of Freya is always a good one and always easy to modify. I even still see it on store shelves now and again.
Nemoren's Vault is OOP (out of print) but very good, after some re-writing and re-thinking. I had fun with it anyway. May have helped that I ran my kids through it.
Other than that, I haven't really bought any low-level modules. I used to subscribe to Dungeon. Technically, I still do since my 3 year subscription hasn't expired yet. should at any time now.
I'll second the accolades for Crucible of Freya. I just finished using it & the free download The Wizard's Amulet (as a percursor) to kick off a new campaign. Eventhough the organization of Crucible is whacked and there's probably too many high level encounters (all optional and meant to be used later in your campaign), it's a good story and a fine way to begin a new group at 1st level. I'd seriously consider starting your group off with Wizard's Amulet though since they'll need the experience before moving into the Big Battle. Several of the combatants in that encounter will make short shrift of 1st level characters, so having the party advance to 2nd seems crucial.