Since I upgraded to WordPress 2.3, The biggest thing I missed about my old Simple Tags Plugin was the auto-completion that happened for tags when I started typing them in on a post. I started missing that so much that I began to think of writing a new plugin. Before beginning on such a task however, I wouldn't even think of not searching Google. Well, I came up with the Recommended Tags Plugin. I can't say enough how this is exactly what I was looking for.
From their description:
This is a plugin for WordPress 2.3+ that utilizes the built-in tag system for recommending and displaying a list of your current tags. It avoids the troubling issue of creating duplicate tag names that are nearly identical, such as “download” and “downloads.”
All I was looking for was auto-completion, to help me avoid typing in different spellings or slightly different tags that have the same meaning. This plugin goes way beyond that by listing your available tags and recommending tags based on the content that you've written for your post. It's a must install in my opinion.