Google realized that one of the reasons Google Talk has such a small number of users is that you need to have a Gmail account to use it. And that wasn't too necessary. Of course, you had your Gmail contacts, mail notification, your chats were searchable in Gmail and the voicemail you received looked better, but the basic client doesn't need a Gmail address.
Google Blog reports that now anyone can try Google Talk, if they have a Google Account. So if you're in this situation, you just have to sign up for GTalk, choose a nickname, and get the client (1.5 MB, Windows 2000/XP). You'll notice that the interface still talks about Gmail and features like mail notification are visible, but don't work.
Google claims "there are millions of people around the world to chat with".
Update: Google also released Google Talk 184.108.40.206, fixing some bugs, and introducing others. Offline contacts and those who use Gmail Chat have non-working options like "Call" and "Send files".
Some reasons to try Google Talk
Google Talk tips and tricks
Why Google Talk will be the best IM client