Startup Disk Nearly Full

When I have problems like this, the best approach is to:

• open Finder
• Go to your root directory (Macintosh HD, in most of the cases)
• Chose "list" mode
• Press  Command + Shift + . (or the period)
• Click View > Show View options and click Calculate all sizes
• Click on the "Size"column to order from large to small
• wait...wait...wait.

This will, slow, start showing how much space is in use by each directory. You can click the ">" on the left side to show sub-directories. Ignore System and Library directories.

With this information in hands, you can identify where the space of your Volumes is gone.

Remember that Documents is a link to iCloud. The space used by it can be adjusted to offload files if necessary.

This is how one of my computers looks like.

Seems that the Apple Mail program is downloading a log of messages from your Hotmail account.

Assuming this is a new behaviour., you need to check if you had received some message with a large attachment on it.

PS: Ignore my comment about iCloud. Snow Leopard is not using it yet.