Route 221 - Battle items
There's a house on the northern side of Route 221 (west of the Pal Park) where a man lives who always wants to see Pokemon of a specific level. Once per day, if you show him a Pokemon that's the level he's asking for, he'll give you either a Black Belt, an Expert Belt, or a Focus Sash, in that order. He'll repeat giving you those three items (only one per day, of course) as long as you keep showing him Pokemon.
Above: PH Balance? That really comes up in conversation a lot...
And here's a few extras just for fun - they're not necessarily useful, but still worth checking out.
Sunyshore City - Ribbon lady
In a house in the far northeastern corner of Sunyshore City lives a woman whose passion is ribbons. She gives out one ribbon per day, and will give each of your Pokemon up to seven ribbons total. If one of your Pokemon already has each of the seven ribbons she gives out, she'll ask you to show her another Pokemon.
Snowpoint City - Vocabulary builder
Talk to the man in the northeastern house in Snowpoint City and he'll add another word or phrase to your vocabulary once per day. The vocabulary that you start out with in the beginning of the game is frustratingly limited - so go to him if you want a bigger selection of "trendy" things to say to put in letters to your friends.