I'm not sure if you follow Jill Corcoran's blog (she's an awesome agent with The Herman Agency), but she posted THIS on Sunday:
READ THIS; NOW PLEASE SEND ME MG MANUSCRIPTS:)
1) Click on the link below,
2) Give it a read,
3) NOW PLEASE SEND ME MIDDLE GRADE MANUSCRIPTS:)
LINK: Jill Corcoran Books: LET'S TALK MIDDLE GRADE
Here's a link to Jill's Publishers Marketplace page.
Here's a link to THE HOW'S AND WHY'S OF SUBBING TO JILL CORCORAN, HERMAN AGENCY - she just posted this today, November 16th.
FYI, it doesn't indicate it in this particular post, but I've read in other places that Jill only accepts queries from published authors or SCBWI members. She may have changed her policy, but to look into becoming a member of the Society for Children's Book Writers & Illustrators, click HERE. It's $85 per year, and they have different levels of membership, so you don't actually have to be published to join.
The Herman Agency also represents Young Adult fiction (one of their clients is Janet Gurtler, who wrote THE WEIGHT OF BONES).