Anybody who blogs about programming related topics needs to post code at times.
When I was on Wordpress, I used the WP-CodeBox plugin. It worked quite well, but it was server side. This is fine if you intend to stay with your software forever, but when I moved off Wordpress to my custom solution, it became a problem.
Some downsides include the fact that you have to include the brushes. They are separate, so you can include selectively, or just include all of them. Also, as the docs say, you should replace the '<' character with the HTML equivalent '<'
Pastie is quite popular among the irc folks. You paste some stuff, and it takes care of storing it, and highlighting everything for you. You then just put the embed code in your blog, and away it goes.
The downside for something like this, is that if Pastie is down, your blog hangs while it's trying to load the script. On top of that, your code isn't in the post. It won't show up in RSS feeds (probably, depending on how awesome your reader is), and therefore won't be seen by search engines (good or bad, whatever).
I moved everything from WP-CodeBox to GitHub's gist. I have faith that GitHub isn't going anywhere anytime soon, and despite some minor issues as of late (which will hopefully be remedied by the move off Engine Yard), the scripts load reliably and quick. As an added bonus, gists's are all backed by git, so you can check them out and edit in your own text editor, fork them, and comment on the progress as you change them. Awesome.
The downsides are the same as Pastie.
What is everyone else using?