Would you like a Google hack to bring folder support in Gmail's IMAP implementation, in that case, there's a way to better visualize the nested folders. Basically Gmail converts any folder into a label: if you create the folder Comments as a subfolder of Blog in a mail client, Gmail will convert it to the Blog/Comments label.
To see the folder structure, you can use the Folders4Gmail Greasemonkey script (requires Firefox and the Greasemonkey extension). Instead of Gmail's geeky labels, you'll see the hierarchy, but don't expect that by clicking on a folder's name Gmail will show all the messages from the subfolders. Only the folders that don't have any subfolder are actually useful.
The script can be used even if you don't like Gmail's IMAP, but in this case you'll create parent folders instead of subfolders. If you already have Comments and Tips as two labels in your Gmail account, you need to create the parent folder (the label Blog) and add Blog/ to the name of each of the two labels (Comments becomes Blog/Comments).
Many people want folders or hierarchical labels in Gmail, but this is as close as you can get. Those who really want support for folders will feel more comfortable using an email client instead of Gmail's web interface: you can easily create new folders and use drag&drop to move messages to a folder, even if the folder is actually a label in Gmail's internal representation.
Gmail, IMAP, Gmail IMAP, Folder, Tips and Tricks