How can I recover a crashed hard drive?

My hard drive from my MacBook crashed and died hard last week. I replaced the drive with a new one, and installed 10.6/Snow Leopard on it. The old drive is now in a usb enclosure, and I really need to recover the data from it.

The problem is, it won't mount and when it's connected with USB, OSX doesn't even detect that there's a new drive attached.

What can I do?

Your best bet is to restore from your most recent backup. You have backups, right? :-)

If you don't have backups and the drive isn't even recognized as existing when you plug it in you're probably in Data Recovery Company territory (also see Google: "Data Recovery Services"), and data recovery tends to get hideously expensive.
Out of the 3 linked I would recommend starting with Tekserve -- They deal with Macs regularly, and if they can't recover your data at their facility they may be able to refer you to someone who can.

Realistically unless the data on this disk is of super-critical business importance and was the only copy in the world recovery probably isn't worth the price. If it IS super-critical this is an expensive lesson in why you need to start backing up (If you're using a Mac I suggest using Time Machine: Mostly automatic & a good fit for 99% of users)

If the harddrive is still recognized as a harddrive on the hardware level, you can use ddrescue: http://www.gnu.org/software/ddrescue/ddrescue.html .

I had great success recovering data from a partition that Windows would no longer mount.

ddrescue basically creates an image of a partition, same as any imagin tool. The critical difference is that it will not hang or inifinitely retry on read errors; rather it will skip damaged regions, but remember them and revisit them after copying everything that had no errors. That way, you quickly get the undamaged parts, and then can keep it running until it no longer recovers anything.

Afterwards, you can run fdisk on the image to recover what's there.