There are several ways of building tag clouds for which you'll pay a monthly fee, or have to buy software, but why not make them for free? After some serious hiccups caused by a huge migration from delicious, Diigo has emerged as the best place to build a tag cloud.
To begin, you'll need an account. It's free. The next step is to start tagging your posts with keywords as described here. This creates the database of tags from which your tag cloud is built.
To create your tag cloud, click Tools in the menubar at the top of any Diigo page. Under Web Services on the Tools page, you'll see Tagrolls. Click that. It'll take you to the Diigo Tagrolls page where you can tweak how you want the tag cloud to look. Once you're satisfied, copy the HTML code in the lower left of the page and paste it into a Raw HTML object in your site-building software (Sandvox for me.) This is your tag cloud… and I'd strongly suggest putting it in the sidebar of your blog.