I use Apple's Backup as provided to .mac members. I have it set up to back up everything to an external once a week. It does incremental updates so after the first full back up it only updates what has changed.
Another option is to manually drag the files you want to back up onto the external. This works, but obviously it requires you to remember to back up and to try to remember what has changed or overwrite a lot of files repeatedly. Don't really like this option myself.
Finally, you can use a program like Carbon Copy Cloner that will exactly duplicate the drive you are backing up. If the original drive fails you can then just start using the back up.
This is a just a starting point. I would suggest some time googling to discover the pros and cons for yourself. Easiest of course is to get Leopard and just use Time Machine.
The Apple Backup app still works if you are not a .Mac member. Backing up to .Mac online is restricted to .Mac members but the application works with or without .Mac! I use it to back up to a firewire external hard drive. The first backup session will do everything but after that it is incremental. You can easily select whatever you want to back up but it does take some time to index or whatever it's doing. .Mac or not it seems to work fairly well.
There is a free app called Silverkeeper from LaCie. I haven't used it yet. I don't know if it does incremental backups or not.
For cloning a disk I've heard Carbon Copy Cloner works.