I am using the Visual.Syntax code highlighting plugin by Matthew Delmarter. There were some code highlighter plugins out there, but they were all pretty complicated and the output was fancier – a little too fancy. I wanted something simple to copy/paste from and Visual.Syntax won the day for me. iG:Syntax Hiliter is a close second and I may still use it some day. It does neat things like line numbering and highlighting for a large amount of languages; but I did not find it as simple to use as Visual.Syntax. In case you were wondering, both of them work with WordPress 2.x.