While Gmail is a great web mail service, the way it treats attachments might confuse some people.
In Gmail, you can't send executable files (.exe, .cmd) or ZIP archives that contain executables. To bypass this limitation, you need to rename the file and change its extension (don't forget to mention this to the person that receives your mail).
Although Google says you can't send attachments larger than 10 MB, Gmail is quite forgiving and lets you send files up to 13-14 MB, so you don't have to worry about size. [Update (May 2007): Gmail has increased the maximum attachment size to 20 MB. ] If you need to send bigger attachments or you send your files to a lot of people, consider uploading them to a file hosting site (like DivShare, mihd or QuickSharing) and including the URL in the mail. For documents that require collaboration and reviews, Google Docs is a good solution, while Picasa Web and Google Video are better options if you need to share photos and videos.
It's a good idea to select the files you want to attach before writing your email, because Gmail starts to upload them immediately, saving you precious time. If you want to be reminded to attach a file if you talk about attachments in your email, this Greasemonkey script is fairly good. To upload the files using drag and drop, install this Firefox extension.
Now that you sent your message, you may wonder how to retrieve it in the future. To search for emails that contain attachments, use: has:attachment. If you know some words from the title of an attachment or its extension, add them: has:attachment filename:pdf or has:attachment filename:author filename:review. Unfortunately, the only searchable attachments are text files, so you may want to upload a plain text version of your documents if you need to search their content later.
Google offers you the option to view online a lot of file types: Microsoft Word, Excel, PowerPoint files, PDF, RTF and even edit Word and Excel files using Google Docs. This is a simple way to convert all these file types to HTML. You can also listen to MP3 files directly from Gmail.
While Gmail offers plenty of space (almost 3 GB), it's not a very good idea to use it for storing files. There are tools that make it easy to upload files to Gmail (the most well-known are the Firefox extension Gspace and the Windows application Gmail Drive), but Gmail was not created for this purpose, so they're just clever hacks. If you upload too many files, Google could even lock your account for 24 hours.