Two useful features have been added to Google Labs recently.
Message Sneak Peak allows you to preview a message from your Inbox by right-clicking on it without having to load the whole message page. This will be great when searching for specific content in a list of e-mails, for example, from people who refuse to write subjects for each message.
Nested Labels is Gmail's first attempt at allowing users to store e-mail in a folder-like structure. You can apply this by using the following structure when naming your labels: "Folder/Label" and even "Folder/Subfolder/Label". This is still rather rudimentary as the label will show up in your inbox as "Folder/Label" but is certainly a step in the right direction.