The best place to watch Smallville online without downloading is Amazon Prime. They have all 10 Seasons available, and several options as to how you can watch. You can buy an episode to watch online for £1.89 in SD or £2.49 in HD.
There are also ongoing subscription charges associated with Prime though, so if you were looking for a free option, you may wish to consider one of the free online streaming sites.
The best site I can recommend is Watch Series. As it stands, it looks as through they have valid links for every episode of Smallville- hurrah!
Watch Series is a site that provides links to content, it does not actually host the content itself. This means that you have go through several different screens to actually get to the episode, and there are also a lot of pop up ads you need to navigate your way through, so be aware of that before you start.